INPUT, OUTPUT, WHAT'S THE EXPRESSION?????

When talking about input and output, I use a wood chipper to help students understand the concept. We have all seen a wood chipper at some point in our lives and know that a tree limb goes in the machine, something happens to it and out come wood chips. This is the same for input and output. We substitute a value for the variable (tree limb), use the order of operations to simplify the expression (wood chipper), and then we get the answer (wood chips).