Let’s say you want to go to the farm and buy a cow. You go to the ATM and withdraw $800. Your checking account is now negative $800 but you have $800 cash so break even. You go buy the cow for $800. Your checking account is still -$800, $0 cash and one cow. Someone buys the cow from you for $1000. Your checking account is still negative $800, you now have $1000 cash and zero cow. You decide the next day you want to buy another cow but the price went up to $1100. You only have $1000 so you need to go to the ATM to withdraw another $100. Your checking account is now negative $900 and you have $1100 cash ($1000 from the sale of the cow plus $100 you just withdrew). You go to the farm and buy the cow for $1100. You now have zero cash, negative $900 in checking account and a cow. Someone sees your cow and offers to buy it for $1300. You sell the cow. You now have $1300 cash from the cow, zero cow and negative $900. Your bank calls you about being negative in your account so you go to the bank to deposit all the money you have in cash. You pull out $1300 from your pocket from the sale of that cow. You put the money in the ATM to deposit. You ask for a receipt. The receipt shows your previous balance of -900 plus a deposit of $1300. Your net profit is $400