[Seaman Kids]

Seaman Kids  asked the Naked Scientists:

Every sunday evening my dad has a gin and tonic with lemon and my mum has a rum and coke with a piece of lime.

But the lemon floats and the lime sinks! Why is this?
   
  Amy Seaman

What do you think?

[lyner]

If an object floats in a liquid then you know its average density is less than the liquid and vice versa.
In the case of lemons and limes this may be due to the skin of a lemon having air spaces in it but the lime skin may not and, consequently, be a bit more dense.
It is possible that the bubbles of one fizzy drink tend to attach to lemons but the other fizzy stuff doesn't stick bubbles to lime . Look closely; are there visible bubbles on the outside of the lemon slices?

[paul.fr]

If an object floats in a liquid then you know its average density is less than the liquid and vice versa.
In the case of lemons and limes this may be due to the skin of a lemon having air spaces in it but the lime skin may not and, consequently, be a bit more dense.
It is possible that the bubbles of one fizzy drink tend to attach to lemons but the other fizzy stuff doesn't stick bubbles to lime . Look closely; are there visible bubbles on the outside of the lemon slices?

you may also want to try this:
http://www.thenakedscientists.com/forum/index.php?topic=6730.msg175827#msg175827