Formulas turn to values

This HAS to be a basic question, but it just drives me nuts.

Every so often I get a spreadsheet form some one else, and when I
start to edit it formulas turn immediately into values.

For example, I enter a =10/2 ,when pressing enter, the cell displays correctly
the result, 5. But when I click into the cell, instead of having the formula
=10/2, the only value inside is 5.

How do I keep this happening ???
I want the cells to keep the formulas.

Really baffled.
THANKS FOR YOUR HELP