When I was 4, my sister was 2. I am now 44. How old is my sister ?

This is a tough one. She might be 42, but she could be 41 or 43 also, since you don’t say when your birthday is and her birthday is. Also, she could be dead. Finally, you might have thought she was your sister but actually your mom had an affair with another man and the woman who is 1–3 years younger than you is not actually your sister. But your mom may have given away your real sister for adoption, in which case she’s probably older than you. See why it’s tough?

Finally, what if you think your sister is dead, but she actually has become an astronaut on a secret government project for near light-speed travel? Then your sister would be aging more slowly than you, so she could potentially be even younger than 41.