What time is it when the clock strikes thirteen?

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?