Question: There is a challenge involving a lemon floating in a jug of water which seems impossible to beat. I've noticed it in several pubs of Edinburgh.

The challenge is as follows:

* There is a jug half filled with water.
* Floating in the water, there's a lemon. The lemon doesn't touch either the bottom nor the edges of the jug.
* The challenge is to successfully balance a coin on the lemon.
* Modifying, moving, or more generally touching the lemon are not allowed.

Any attempt to balance a coin on the lemon seems to result in the lemon flipping over, and the coin to sink in the water.

Why is it so hard to balance the coin while it's extremely easy to balance a coin on a lemon set on a table? How do you beat the lemon challenge?

Answer: 
[![enter image description here](https://i.stack.imgur.com/IKbME.jpg)](https://i.stack.imgur.com/IKbME.jpg)

The real answer is a trick. Sorry

Take a **heavier** coin and squeeze it in sideways underneath, so it's now a lemon with the centre of gravity at the bottom, like the keel on a sailboat.

As long as the top coin is small and light, it should balance.
I don't happen to have a lemon at the moment. But here's what I would try. I would try to place the coin so that it's position is as close as possible to the water plane. So then near either tip of the lemon rather than the center. My guess is the lemon is somewhat more stable in pitch than in roll, so it may not pitch up. And if the coin is closer to the water plane the disturbing torque in the roll axis may not be enough to turn the lemon.