Why is Saturday stronger than Monday?

I make a loud sound when I’m changing. When I do change, I get bigger but weigh less. What am I?

I am a type of word that is spelt incorrectly or used differently, on purpose for humour. What am I?
Hint 1: You can find me in a joke.
Hint 2: I am a play on words.

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

There is a box full of marbles,
all but two are blue,
all but two are green,
and all but two are red.

How many marbles are in the box?

Can you think of any three-dimensional shape that comprises of exactly two surfaces?

Can you find a rabbit in the given picture?

How can you take 2 from 5 and leave 4?

It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?

If 21x = 79x, what is the value of x?