FOUR RIDDLES

1. What happens when Hercule Poirot changes his mind?

2)
The man who made it did not need it.
The man who bought it did not want it.
The man who used it did not know it.

3)
Like a watchman on a bridge,
I protect you.
You can see right through me,
But others wonder what I conceal.

4)
When I walk, it walks.
When I run, it runs.
When I scream, it does nothing.
When I wear colored clothes,
Its color doesn't change.

Answers:

1. A Belgian waffles
2. A coffin 
3. Sunglasses
4. Shadow

Credit:
#1: Robert O. Dillman