money

Yesterday i went to office depot to get some envelope and my dad payed with a $20. When he gave to cashier the 20 she started to look at it for a long minute or so like she had never seen such a thing. When we left my dad and i started joking about it and then i asked him why she really was doing that. He told me it was to make sure it wasn't counterfeit money(fake money). He said there are little threads in the money and when i looked at it there were red and blue threads in it so it had to be real. So that's what i learned about money that i never knew.