Why do we preferably have round manhole covers and not square ones?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

What is the riddle which can be asked all day with a different correct answer each time.

Which tank will fill first?

Which is the rope you never skip with?

I have eight fingers and two thumbs but I do not live. What Am I?

If I put in one canary per cage, I have one bird too many. If I put in two canaries per cage, I have one cage too many. How many cages and canaries do I have?

If 2 workers can complete painting 2 walls in exactly 2 hours.

How many workers would be needed to paint 18 walls in 6 hours?

Jim and Sarah are in a long-distance relationship. Jim buys an engagement ring for Sarah and wants to mail it to her. Unfortunately, the only way to ensure the ring will be received is to place a lock on the package. Jim has locks and Sarah has locks, but neither has keys for each other’s locks. How can they make sure the ring isn’t stolen?

What's the next in the series?

H, Be, F, S, Mn, Kr, In, Gd, Tl, ?