tithing

the neighbors replaced their roof, as i've mentioned before (i think). (we asked if we could have the wood shingles for the furnace. free wood? why, thank you!) taylor was told that if he went around their yard using the large magnet and picked up all the stray nails, they would pay him $2. when he told me, i reminded him that he needed to be sure and pay tithing on it. his response left me a little baffled. he told me that he wasn't going to waste money he just got. from that i deduced that his understanding of the law of tithing, was incomplete.
today taylor and elizabeth came home showing me the money they got from helping another neighbor move rocks. when i told elizabeth that she needed to pay tithing on it, she was also quite upset. she didn't want to let go of her money. then she said she would keep one of her quarters, and pay the other one for tithing.
i need to find/come up with a good lesson on the law of tithing. what it means, how we figure out the amount, and why we pay it. guess it's time for some research!