ONLY FOR GENIUS:
10 + 10x0 + 20x1 + 5x0+ 3
¿Can you solve this?

It's only visible when it's cold. It gets harder when it's higher. Its presence is often unnoticed, but you'll always notice you absence.
What is it?

I go to school with no pen and return home with a pen.
What am i?

Why is it dangerous to put the letter "M" into the refrigerator?

It starts with f and ends with k, is a four letter word and in absence of this you use your hand.
What word is?

I come in a lot of different sizes. Sometimes, I drip a little. If you blow me, it feels really good. 
What am I?