What does below chemistry rebus means?

What happened when the wheel was invented?

What common phrase is represented by the below lines

Easy going:
Weak, 'I'm going.'
Tough, 'I'm staying.'

Medium going:
Weak, 'I'm going.'
Tough, 'I'm staying.'

Tough going:
Weak, 'I can't do it, I'm staying!'
Tough, 'Let's get going.'

Take 9 from 6, 10 from 9, 50 from 40, and leave 6.

How Come ??

A man has to get a fox, a chicken, and a sack of corn across a river.

He has a rowboat, and it can only carry him and one other thing.
If the fox and the chicken are left together, the fox will eat the chicken.
If the chicken and the corn are left together, the chicken will eat the corn.

How does the man do it?

What will be the best approach to finding all the prime numbers less than 75 that leave an odd reminder when we divide them with 5?

Replace the question mark with the correct number.

I get hot but I never sweat,
I cook food but I am not a chef,
I have a door but you can't enter me,
I have a light but I am not a lamp.
Who am I?

Forwards I am heavy, backward I am not. What am I?

There is one thing that goes round the house and also inside the house but never even share a brief touch. What is it?