What famous saying does this rebus mean?

COME SERVED <---
COME SERVED
COME SERVED
COME SERVED
COME SERVED

Most people think of me as money. But when they find me in the water, they won’t get any money out of me. What am I?

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?

Readers with excellent focus can find three differences between the camping scene pictures in 16 seconds. Can you? Test your eye power now!

To attend world science seminar, A famous chemist from Mexico visit
London. In the lab, the scientist was killed and his six assistants were under suspicion of killing the chemist.
Name of six assistants: Austin, Wayne, Dege, Oscaru, Lingard, and Rojo.
He left a note "76-20-44 79-16-22-7"

A local detective Jack was called to solve the case,

After reading the note, Jack instantly asked the police to arrest the murderers.

Who are the murderers?

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

You are given four results as below:
1111 = F
2222 = E
3333 = T
Then, can you find out how to code 4444?

If,

1 = 5 ,
2 = 25 ,
3 = 325 ,
4 = 4325

Then, 5 = ?

There is something which is always coming but never arrives?

The teacher told the student that if he told a lie then he will be expelled from school and if he told the truth then he still is expelled from school.

What can a student say to prevent his being expelled from school?