You Ask someone to borrow 20 dollars. You take their 20 dollars, and put it in the matchbox also You put your own 20 dollars in the matchbox.

You tell them that you will sell them the matchbox for 30 dollars.

If they do it, you just made 10 dollars. you only put in 20 dollars, and they paid you 30 for it.

Walk away before they figure it out!!