"jacked up"

jacob was so proud of himself the other day, and i was proud of him too.  he had his first lesson in couponing and found a coupon for a toy he wanted, so we went to wal-mart.  he had gone home, gotten the money out of his piggy bank, and off we went.  well, we got there, found the toy, proceeded to the checkout and when we got there, daddy gave him the change for the tax that we had figured out.  jacob went ahead of us, very proud and placed his toy on the counter and gave the girl his coupon and the money and when she rang them up she told him the total.  it was more money than he had, so i looked at her and said "you must be wrong, it should be $10.70... $10.00 plus tax."  she assured me she had wrung up the toy, then the coupon and it gave her that total...$11.05  well, daddy gave jacob more money and we went out the door.  i looked at his receipt and figured they charge you tax on the full amount, not the discounted amount, so i said..."well, that's jacked up!"  jacob without missing a beat says, " mom, how did you know his name?"