mold

Which Means:

A container that makes a shape when something is poured in it.

As In:

Heidi made Jell-O in a mold and this is the shape it took.

Heads Up!

'Mold' also means the fungus that grows when something is decaying: Hopefully your Jell-O mold isn't full of mold! In England they spell this word 'mould'.