What property of a molecule defines how it tastes?

Answer is: The shape of the molecule fits into a complementary pocket on the taste receptor protein.
When food (sugars, proteins, fats) or other substances (salt, carbonated water) enter the mouth, molecules interact with saliva and are bound to taste receptors.
There are 5 tastes: sweetness, sourness, saltiness, bitterness and savoriness.

12 is probably the typical age for a first kiss but i think its to young.