It is impossible to drop a round lid into a smaller round hole.
It is because round is the only shape that if turned or flipped over, it still retains the same
length across its center. If you used a square, rectangle, triangle, hexagon or any other shaped lid,
it could be rotated and fall into the hole.
Other reasons
A round cover can be moved by rolling it on its edge.
A round cover contains less metal than a same-sized cover of any other shape.