# Why are man hole covers round? **URL:** \ **Category:** NYSpeed Off Topic\ **Created:** [January 17, 2008, 11:49am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996 "2008-01-17T11:49:51Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![LZ1](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@LZ1](https://forums.speedlife.net/u/LZ1)\ **Post date:** [January 17, 2008, 11:49am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/1 "2008-01-17T11:49:51Z")
I was at an interview in DC a couple weeks ago and this question came up. It’s a popular interview question used at Microsoft So with out looking up the possible answer post up… ---
**Author:** ![ubengineering](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/ubengineering/32/5171_2.png) [@ubengineering](https://forums.speedlife.net/u/ubengineering)\ **Post date:** [January 17, 2008, 11:51am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/2 "2008-01-17T11:51:37Z")
pointy shapes make me insecure ---
**Author:** ![Fry](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/fry/32/7284_2.png) [@Fry](https://forums.speedlife.net/u/Fry)\ **Post date:** [January 17, 2008, 11:52am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/3 "2008-01-17T11:52:02Z")
So they can’t fall in. ---
**Author:** ![Zong](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/zong/32/7580_2.png) [@Zong](https://forums.speedlife.net/u/Zong)\ **Post date:** [January 17, 2008, 11:52am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/4 "2008-01-17T11:52:54Z")
> [@Fry](#): > > So they can’t fall in. x2 ---
**Author:** ![Nikuk](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/nikuk/32/7254_2.png) [@Nikuk](https://forums.speedlife.net/u/Nikuk)\ **Post date:** [January 17, 2008, 11:54am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/5 "2008-01-17T11:54:18Z")
strength @ edges / cant fall in ---
**Author:** ![Fry](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/fry/32/7284_2.png) [@Fry](https://forums.speedlife.net/u/Fry)\ **Post date:** [January 17, 2008, 11:55am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/6 "2008-01-17T11:55:19Z")
My father in law used to interview engineers and would ask: You’re in a little boat in a swimming pool. There’s a big rock in the boat with you. You throw the rock overboard into the pool. Does the water level of the pool go up, down, or stay the same? ---
**Author:** ![RuBiCaNT](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/rubicant/32/7622_2.png) [@RuBiCaNT](https://forums.speedlife.net/u/RuBiCaNT)\ **Post date:** [January 17, 2008, 11:55am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/7 "2008-01-17T11:55:45Z")
there are two reasons. The first has already been posted. The second is because “man-holes” ARE round. ---
**Author:** ![JayS](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/jays/32/15375_2.png) [@JayS](https://forums.speedlife.net/u/JayS)\ **Post date:** [January 17, 2008, 11:55am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/8 "2008-01-17T11:55:51Z")
> [@Fry](#): > > My father in law used to interview engineers and would ask: You’re in a little boat in a swimming pool. There’s a big rock in the boat with you. You throw the rock overboard into the pool. Does the water level of the pool go up, down, or stay the same? Stays the same. And they’re round because it’s the only shape that no matter how you turn it you can’t cause it to fall through the hole. Plus, where would TV comedy be without the fat guy who can’t quite fit through the round opening skit? It wouldn’t be nearly as funny with a square hole because he wouldn’t fill it completely. ---
**Author:** ![jay940](https://avatars.discourse-cdn.com/v4/letter/j/45deac/32.png) [@jay940](https://forums.speedlife.net/u/jay940)\ **Post date:** [January 17, 2008, 11:56am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/9 "2008-01-17T11:56:02Z")
because manholes are round ---
**Author:** ![boardjnky4](https://avatars.discourse-cdn.com/v4/letter/b/96bed5/32.png) [@boardjnky4](https://forums.speedlife.net/u/boardjnky4)\ **Post date:** [January 17, 2008, 11:56am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/10 "2008-01-17T11:56:34Z")
> [@Fry](#): > > My father in law used to interview engineers and would ask: You’re in a little boat in a swimming pool. There’s a big rock in the boat with you. You throw the rock overboard into the pool. Does the water level of the pool go up, down, or stay the same? how would you answer that? ---
**Author:** ![Fry](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/fry/32/7284_2.png) [@Fry](https://forums.speedlife.net/u/Fry)\ **Post date:** [January 17, 2008, 11:58am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/11 "2008-01-17T11:58:52Z")
The water level goes down. A buoyant object (like a boat/rock/person system) will displace an amount of water equal to its weight. A non-bouyant object like a rock will displace an amount of water equal to its volume. Since the rock is denser than water, an equal weight of water is more volume than the rock. ---
**Author:** ![OfficerK](https://avatars.discourse-cdn.com/v4/letter/o/4da419/32.png) [@OfficerK](https://forums.speedlife.net/u/OfficerK)\ **Post date:** [January 17, 2008, 11:59am UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/12 "2008-01-17T11:59:09Z")
plus they are easier to manufacture/press… hrmm this would be a good thesis paper… ---
**Author:** ![rdandy5875](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/rdandy5875/32/7511_2.png) [@rdandy5875](https://forums.speedlife.net/u/rdandy5875)\ **Post date:** [January 17, 2008, 12:02pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/13 "2008-01-17T12:02:23Z")
I’d say it’s because there isn’t one specific point of weakness. Best load distribution. ---
**Author:** ![ILCisDEAD](https://avatars.discourse-cdn.com/v4/letter/i/a87d85/32.png) [@ILCisDEAD](https://forums.speedlife.net/u/ILCisDEAD)\ **Post date:** [January 17, 2008, 12:04pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/14 "2008-01-17T12:04:45Z")
> [@Fry](#): > > The water level goes down. A buoyant object (like a boat/rock/person system) will displace an amount of water equal to its weight. A non-bouyant object like a rock will displace an amount of water equal to its volume. Since the rock is denser than water, an equal weight of water is more volume than the rock. umm… the water level would stay the same… edit: thinking into it more… I guess the density of the rock would come into play ---
**Author:** ![OfficerK](https://avatars.discourse-cdn.com/v4/letter/o/4da419/32.png) [@OfficerK](https://forums.speedlife.net/u/OfficerK)\ **Post date:** [January 17, 2008, 12:05pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/15 "2008-01-17T12:05:11Z")
heheheh sweet Can a race car lift a manhole cover? Modern racing cars create so much vacuum due to their aerodynamics that they can lift a manhole cover off the ground. During races on city streets, the manhole covers must therefore be welded down to prevent injury. In 1990, during the Group C World Sportscar Championship race in Montreal, racer Jésus Pareja’s car struck a manhole cover that was lifted by the ground effect of the car he was following, causing his car to catch fire.[5] ---
**Author:** ![Willybeen](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/willybeen/32/7351_2.png) [@Willybeen](https://forums.speedlife.net/u/Willybeen)\ **Post date:** [January 17, 2008, 12:05pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/16 "2008-01-17T12:05:24Z")
> [@Fry](#): > > The water level goes down. A buoyant object (like a boat/rock/person system) will displace an amount of water equal to its weight. A non-bouyant object like a rock will displace an amount of water equal to its volume. Since the rock is denser than water, an equal weight of water is more volume than the rock. i dont know if i agree with that… volume is what causes the displacement… if the object is more dense than the water it will sink and cause the water to displace its volume… i think there would be extremes at both ends, if the item in the raft (im picturing an inflatable raft) and a rock for one extreme and some composite lighter weight object (that would still sink) as the other… meh im quitting there… i dont feel like typing… man hole covers are round so they dont ever fall in… /thread ---
**Author:** ![boardjnky4](https://avatars.discourse-cdn.com/v4/letter/b/96bed5/32.png) [@boardjnky4](https://forums.speedlife.net/u/boardjnky4)\ **Post date:** [January 17, 2008, 12:05pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/17 "2008-01-17T12:05:33Z")
another good question is: “your from Buffalo I see, how many gas stations are there in Buffalo?” ---
**Author:** ![LAFENGAS](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/lafengas/32/7573_2.png) [@LAFENGAS](https://forums.speedlife.net/u/LAFENGAS)\ **Post date:** [January 17, 2008, 12:06pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/18 "2008-01-17T12:06:02Z")
> [@jay940](#): > > because manholes are round HAHA! YES! if manholes were square, their covers wouldn’t be circular. ---
**Author:** ![LZ1](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@LZ1](https://forums.speedlife.net/u/LZ1)\ **Post date:** [January 17, 2008, 12:06pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/19 "2008-01-17T12:06:42Z")
This pissed me off because we were talking about something else…and then the people interviewing me go by the way why are man hole covers round… _pause_ ---
**Author:** ![newman](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/newman/32/7569_2.png) [@newman](https://forums.speedlife.net/u/newman)\ **Post date:** [January 17, 2008, 12:08pm UTC](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996/20 "2008-01-17T12:08:38Z")
fry is correct about the boat. [Next page](https://forums.speedlife.net/t/why-are-man-hole-covers-round/41996.md?page=2)