Tim and Adam made a bet.Tim bet Adam that he could put something into his barrel to make it lighter. They first weighed the barrel. Then Adam left the room while Tim pondered a solution.One hour later, Adam returned, and they weighed the barrel once again. The barrel was indeed lighter. What did Tim put in the barrel?
The question could have been better if it had been mentioned that
"the thing which Tim had put into the barrel is weightless and doesn't has any shadow".