sign with the name of the variable on it. When you type something like X=1+2 then the computer will calculate the math and place the result in the drawer. X=X+1 means, if X is 5 for example, that you take X, see that it´s '5'. Now it will add '1' and get 6, now 6 gets put into the drawer again. I know this all can be complicated, but if you really think about it, you will get every problem solved!