I used to run a fast food restaurant in Texas. It's correct that the ones running the cash registers just have to put the information in and it will tell you what amount to give back. Having said that, I will say that many customers purposely tried to confuse the cashier in an attempt to short change them. We were getting hammered by these "professionals" until the night we had a couple walk up to the drive through and order a small coke. I had already instructed our teen help to watch for this very thing and to ask me to come and take charge. Sure enough, there was a nice looking couple standing there asking me for a small coke. The guy pulls a roll of 20's out of his pocket and gives me a 20. I do the transaction and give him his small 69 cent coke. He starts to walk away when he turns to me and tells me that he has the exact change and that I should give him back his 20 in exchange for the exact amount. My response was that I would void the transaction and start all over. He knew I was on to him and he immediately started cursing me. As he was walking away and cursing me I told him that we were tired of getting ripped off and to tell his friends that we weren't taking it anymore. Word got out that we were on to them and the amounts that we lost every day went down. I did find it funny when my wife thought she was smarter than they were(and I agree she is pretty smart)and decided to take on two quick changers as they came to two different windows(red flag). Sure enough they both ordered small cokes(red flag) and pulled out big rolls of 20's and each payed with 20's(red flag, red flag). I kept telling her to tell them we were closed but she had to find out who was smarter. She took care of them both and immediately put the closed signs on. As she was counting the registers, I was telling her that she was at least 20 short. I guess I was the smart one because I was right. OH Yeah it's good being right sometimes. To this day she wont listen to me.