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.'

A boy collects white seashells from the sea and brings them home every night. When he has enough of them, he decides to sell them to a trader.

The trader is ready to buy the shells and he asks the boy about the quantity. At this, the boy starts calculating. He has a giant box that contains 3 mini boxes. Two of them have another mini box inside. If the giant box can hold 50 shells, how many brown shells can he sell to the trader?

What's the next in the series?

H, Be, F, S, Mn, Kr, In, Gd, Tl, ?

What four weights can be used to balance from 1 to 30 pounds?

If you say my name, I will no longer exist. What am I?

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

Who am I?

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

A car is crossing a 20 km-long bridge. The bridge can support at most 1500kg of weight over it. If somehow, the weight on the bridge becomes more than that, it will break.

Now, the weight of the car is exactly 1500kg. At the midway, a bird comes and sits on the roof of the car. This bird weighs exactly 200 grams.
Can you tell if the bridge breaks at this point or not?

By what process could you make a 'Tea-Table' into food?

The more you take me, the more you leave me behind.
Who Am I?