Here's a letter I send to each kid in the class:

Dear Kids,

I am looking forward to visiting your class. I hope you are really good at counting,
because I need your help. I would like you to help me figure out how many hairs
there are on my head. I don't think we'll have time to count all the hairs, so maybe
we should try to estimate the number instead? But how?

By the way, if you turn this letter over, you will see quite a few dots on the back. How
many dots do you think there are? I KNOW THE ANSWER! If it sounds like fun,
try and come up with a good estimate before I visit your class.

 

Sincerely,

Will Kazez
Professor of Mathematics

Here's the back of the letter. Maybe you'd like to try it?