The part of the cherry everyone throws away

What a cherry pit actually is

A cherry pit is mostly lignin — the stiff polymer that makes wood woody. It is dense, it is dry, and it takes on heat slowly and gives it back slowly. That is the whole reason anyone fills a heat pack with pits instead of rice.

The second reason is shape. Pits are irregular, so a filled pack traps air between them and drapes over a shoulder instead of sitting on it like a brick.

This site is about that pit: what it does, where it comes from, and how to use it. The heat figures are here.