Food in a hydra...
What must cells be able to do?
...cells lining stomach release an enzyme to break down food molecules
...grab smaller molecules produced by digestion, move them inside
...release some of those molecules for other cells to use, too (e.g. tentacles)
...all cells re-configure those molecules to make hydra cell parts
There must be different cell types to do different functions, but they all have the same basic requirements.