assigning a unit ?

Hi. I'm sorry, this is probably the most basic question ever asked here, but for some strange reasons I don't find anything in the help file or on the web...

I just want to assign a unit (e.g. centimeter) to a value in a cell, let's say:
7 cm

I found plenty of explanations concerning the conversions of units, but none that explains how to assing a unit in the first place...
Logically, I would have expected that the user can assign units the same way as he can choose the cell format (currencies: dollars, euros, ...; time and date; percent; etc.), but when I right-click on a cell and choose "cell format..." (or whatever that translates to in the English version of Excel) there is no option for units (other than currencies) ...

Any help would be most welcome.
And sorry for being such a newbie ...