The Student Room Group
Reply 1
They contain some mineral which upon the contact of water starts to effervesce and produce CO2 which breaks up the tablet.

[EDIT] Having done some research at least one brand of tablets use citric acid and sodium bicarbonate. As the tablets dissolve the acid and bicarbonate react vigorously producing carbon dioxide gas, which also produces enough agitation to allow the active ingredients to dissolve readily.
Reply 2
Very cool. Cheers.
Reply 3
Hah, that's the same composition as sherbet. Without the sugar or whatever the Hell else is in sherbet.