What goes up but never comes down?

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?

Can you find the next number in the below sequence
3 , 7 , 10 , 11 , 12 , 17 ?

Which is the smallest number that you can write using all the vowels exactly once?

You have 10 balls with you. A friend of yours out of nowhere asks you to place those ten balls in five lines such that each of the lines has exactly 4 balls on them. He needs to check your intelligence. Prove him by doing the task.

What is always in front of you but can’t be seen?

In this image, you can see three almost identical Doraemon images. Can you spot the odd one out from them?

You have two strings whose only known property is that when you light one end of either string it takes exactly one hour to burn. The rate at which the strings will burn is completely random and each string is different. How do you measure 45 minutes?

It is a nine-letter word.
The exact opposite of the phrase "friendly and outgoing"
It contains the word "over".
It starts will a vowel.

What is it?

How can you make the following equation true by drawing only one straight line: 5+5+5=550 ?