Back-of-the-Envelope Calculations: Surface Area of the Moon vs. Earth
If you could wrap the Moon in a gigantic cloth and then unwrap the cloth and spread it out on the Earth, how much of the Earth's surface would it cover?
References and Resources
This SERC page describes the use of Back of the Envelope Calculations
The Back of the Envelope : This page outlines one of the essays in the book "Programming Pearls" (ISBN 0-201-65788-0). The book is written for computer science faculty and students, but this portion speaks very well to back of the envelope calculations in general.