Chemist H.S. Paine invented the liquid center chocolates in 1924 by exploiting the different solubilities of three sugars. Common sugar, sucrose, is a combination of two simpler sugars: glucose and fructose. A mixture of equal parts glucose and fructose is very soluble in water; a large amount will dissolve without forming solid crystals. Sucrose is less soluble, and an equivalent amount of sucrose in water forms a paste-like solid (fondant).
Paine knew that many organisms consume sucrose, and that they first break it into the simpler sugars by using an enzyme.
sucrose + water → glucose + fructose + water