I call the function machines gumball machines instead. I drew a funny looking gumball machine with lots of lines/tubes going back and forth like those that go round and round.
The more rows the more the gumballs you get (+) or loose (-). If they loose them (-) I say the gumball machine kept them, if they add (+) they are given more :) They actually got it. :)
Later, we went to the computer lab and did this function machine from Johnnie's Math Page.