adding numbers in a single cell as a running total in that cell ?

Hello everyone, I am new to this forum and have been using excel for about 3 years. I have learned alot but still can not figure out how to make numbers add up in one box like a running total. Meaning, If I have a list and in the list is one cell for each item say like cell a1 is for a can of coke and cell a2 is for a car tire and so on,and i want to keep a running total by adding a number to cell a1 or a2 like say "3 i.e. 3 tires or cans of coke" and later that day i come back and need to add 2 more ......so i want to be able to just click on the cell a1 or a2 and enter the number 2 and the cell would add the number in it to the number im adding to it to show a result of 5 tires or whatever. I am sorry If i am not asking the question in an understandable manor but its the best way I can think to ask it.lol anyways, any help would be appreciated, Thanks in advance.


James