What is so fragile that saying its name breaks it?

An express train takes 3 seconds to enter the tunnel which is 1 km long.
If it is travelling at 120 km an hour, how long will it take to pass completely through the tunnel?

What is black when you buy it, red when you use it, and gray when you throw it away?

What three numbers, none of which is zero, give the same result whether they’re added or multiplied?

Buy me, smell me or deliver me, I do not change.

Who am I?

No matter how little or how much you use me, you change me every month. What am I?

You see me once in June, twice in November, and not at all in May. What am I?

The Puzzle: A girl, a boy, and a dog start walking down a road.

They start at the same time, from the same point, in the same direction.

The boy walks at 5 km/h, the girl at 6 km/h.

The dog runs from boy to girl and back again with a constant speed of 10 km/h. The dog does not slow down on the turn.
How far does the dog travel in 1 hour?

I am an eight letter word and I am a computer terminology.
The second, third and fourth letters make an animal.
The fourth, fifth, sixth, seventh and eighth letters make a weapon.
The first, second, third and fourth letters can be taken as an outcome of any exam.
The fifth, sixth, seventh and eighth letter combine to form a high end typing software.

Can you tell who am I ?

What can you deduce from the following BrainBat?

CRY
LMKI