Difficult clever riddles for teens with simple answers

What is the ancient invention that allows people to see through walls?

It always becomes white when it becomes dirty. What is that?

What always sleeps with his shoes on?

I have more than 80 keys, but I cannot open any lock. What Am I?

What you cannot hold it but it is yours?

I drift forever with the current down these long canals they've made. Tame, yet wild, I run elusive, multitasking to your aid. Before I came, the world was darker. Colder, sometimes, rougher, true. But though I might make living easy, I'm good at killing people too.

Whoever makes it, tells it not. Whoever takes it, knows it not. Whoever knows it, wants it not.

I am something to make you think, I am something to make you know. I am something to which the answer you might never know. What am I?

On the first day they cover one quarter of the total distance. The next day they cover one quarter of what is left. The following day they cover two fifths of the remainder and on the fourth day half of the remaining distance. The group now have 14 miles left, how many miles have they walked?

When you have me more, you can see only less. What Am I?