Uncle radi Truck

Uncle Radi has a truck and he transports rocks everyday. He wakes up in the morning and fills his truck with 40 rocks and transports them to get paid.
His wife said the truck that can hold 40 rocks can hold 50 so he added the amount. The next day his son said the truck that holds 50 rocks can hold 60 so he added the amount. The day after his neighbor said the truck that holds 60 can hold 70 so he added the amount. On the way the truck broke and he had to take it to be fixed and he paid all the extra money he got from holding the extra rocks on fixing the truck. Now he knows that he should transport only what his truck can handle and being greedy is harmful .