Kalvin tosses a coin five days in a row and gets tails everytime. Do you think there is something wrong with the coin? How can you find out?

There is probably something wrong because it should have been tail half and heads half the time.

The probability of that happening is only. 1/32 ... pretty small, but not impossible. I wouldn't blame the coin though, until he tosses it 100 or 200 times, and gets a split that's wildly different from 50-50.