Why do we say tummy instead of belly or stomach?


Tummy is an informal word we use with little children. e.g. "Does your tummy hurt?"
Belly is the abdominal area (usually for adults). e.g. As we get older, and eat too much, we will get belly fat.

Stomach is the internal organ that digests food.


"Tummy" comes from baby-talk for "stomach". People often use baby-talk with infants, and some of it has made its way into the language.

We informally refer to the belly as the stomach. You can get punched in the stomach (in the belly), and you can lie on your stomach (lie prone).