How can you make number one disappear by adding something to it?

Two guards were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?"

How did he know his companion was smiling?

I am the one who remains awake when you sleep. But I need to rest when you awake. Who Am I?

The product of three consecutive numbers is 7980.
Then the sum of these consecutive numbers would be?

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

Yesterday I fell from 30 feet high ladder but I don't get hurt. Why?

I come in different shapes and sizes.
Parts of me are curved, other parts are straight.
You can put me anywhere you like,
but there is only one right place for me.
What am I?

I am a type of room you cannot enter or leave. Raised from the ground below, I could be poisonous or a delicious treat. What am I?

You have four chains. Each chain has three links in it. Although it is difficult to cut the links, you wish to make a single loop with all 12 links. What is the fewest number of cuts you must make to accomplish this task?

What animal walks on four legs in the morning, two legs during the day, and three legs in the evening?

Oedipus, the king of Thebes, figured out the answer to the riddle: