Why are man hole covers round?

[rdandy5875]

@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.

+1, read the reply before I though about it myself though.