If you say my name, I will no longer exist. What am I?

In an interview, a boy was asked an unusual question 'How two persons sitting with a table in between them can't see each other?' He was unable to reply. Can you?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

What has many keys but can’t open a single lock?

A monk has a very specific ritual for climbing up the steps to the temple. First, he climbs up to the middle step and meditates for 1 minute. Then he climbs up 8 steps and faces east until he hears a bird singing. Then he walks down 12 steps and picks up a pebble. He takes one step up and tosses the pebble over his left shoulder. Now, he walks up the remaining steps three at a time which only takes him 9 paces. How many steps are there?

Twenty frogs are sitting on a log floating on the surface of a river. Two of them decide to jump off into the water.

How many frogs are there on the log at this moment?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?

John’s parents have three sons: Snap, Crackle, and what’s the name of the third son?

I’m tall when I’m young, and I’m short when I’m old. What am I?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.