Stacking items

Hey all, ive just got a quick question here that i kinda need to know for a map im making.

How do you make items, such as potions, combine charges left when they are picked up? im sure this is relatively simple to you guys, so could ya tell me? ive got a few ideas...but none seemed to work.

thanks, LUEE

Here is a thread about merging items on pickup, with some examples of triggers that do what you are requesting.

thanks Anitarf. thats a great help