"How do you solve this math problem?", the high schooler asked.

"Err...", we looked at the homework. "Logarithms. You have to use logarithms."

"What are logarithms?"

Oh boy. What followed was a painful plunge into the depths of our mathematical knowledge that somehow pulled out a (somewhat) coherent explanation.

"So how do you calculate logarithms?"

"Well, you can find a website that will have a logarithmic calculator..."

"How did you do it before websites?"

"You can use a scientific calculator, only it's not that simple, you need to..."

"What about before calculators?"

"Ummm..." Then we remembered. "Slide rule!"

"What the hell is a slide rule?"

This, kids, is a slide rule.

It's a devise invented all the way back in the 17th century, but not perfected until the 19th, when Jewish French mathematician Amedee Mannheim added that glass runner to it. The "modern" slide rule (and we use that term very loosely!) was even called the "mannheim". The slide rule can calculate logarithms and do all kinds of complex calculations, like trigonometry and roots. If you know how to use it, that is.

"So how does this slide rule work?"

"Ask grandpa!"