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

The person who makes it has no need for it; the person who buys it has no use for it. The person who uses it can neither see nor feel it. What is it?

What is the largest amount of money that you can possess in change and still if someone asks you for a dollar, you won't be able to give?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

All of the flowers I have are orchids except two. All of the flowers I have are hibiscuses except two. All of the flowers I have are roses except two.

Can you tell me how many flowers I have?

If 1+9+8=1, what is 2+8+9?

What do the following words have in common?

Assess
Banana
Dresser
Grammar
Potato
Revive
Uneven
Voodoo

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

What have one eye but still cannot see?

There are hundred red gems and hundred blue gems. The blue gems are priceless while the red gems equal wastage. You have two sacks one labeled Heads and the other Tails. You have to distribute the gems as you want in the two sacks. Then a coin will be flipped and you will be asked to pick up a gem randomly from the corresponding sacks.

How will you distribute the gems between the sacks so that the odds of picking a Blue gem are maximum?