Over Easter, I had a very nice lunch back in south Wales, at The Butchers Arms in Alltwen. It was very good, I can recommend it if you are in that part of the world. We asked for a jug of water, which came with slices of both lemon and lime. Very nice. As a scientist I was curious about the fact that the lemon slice floated on the top of the water, while the lime slice sank to the bottom.

Clearly, the slice of lemon was a little less dense than water while the slice of lime was a little more dense. I suggested to my sister (also a scientist) that maybe the lemon slice floats because it had more pith on it than the lime, and that the softer white pith is the least dense part and so if a slice has a large fraction of this pith it will float.

I should say that I think that lemons and limes are very similar fruits, so it seems unlikely that there is much difference in density between say lemon pith and lime pith.

So I bought a lemon and a lime at Tescos and experimented – see the picture above. I sliced the lemon and lime in different ways to get slices with varying amounts of: rind, pith, and the bit in the middle with the actual juice (sorry, don’t know what that is called).

If you look at the picture, all the pieces are floating except two small bits at the bottom. The left hand one of these is easy to see at the bottom but the other part is smaller and harder to see – sorry. Both of these pieces are mostly rind. So I think the rind is the dense part and that slices should sink if they have a lot of rind, but should float to the top of the water if they have relatively little rind. I then tested this but taking a small piece of lemon rind – which sank.

So I am feeling quite good about myself, I have followed the scientific method. I started with an interesting observation, formulated an hypothesis, tested it with experiment, drew conclusions from this and modified my hypothesis, and then even did a final experiment to confirm my finding.

But it was only one lemon and one lime, if you come across different observations, at home or in a restaurant, comments are open below for two weeks.