Why does this happen? To help figure it out we tried a few tests. First we took the lemon out of the water put it on the table. It was fairly easy to get the coin to balance.

We observed that when the lemon was in the water it started to roll as soon as the coin was placed on it. When it rotated beyond a certain point the coin fell off. On the table the lemon didn’t roll and the coin stayed put.

Then, we put the lemon back in the water and tried a scrap of paper instead of a coin.

The lemon didn’t roll and the paper remained on top.

We wanted to see what would happen if the coin were attached to the lemon. We cut a small slit through the lemon skin and stuck the coin in. We put the lemon in the water with the coin pointing up. It rotated so that the coin was on the bottom, and as it turned, the lemon sank to the bottom.

What do you think is going on? Why did the coin slip off the floating lemon? Are there some other tests that you might try?

Click here for our explanation.

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