# How Many Drops Of Water Are In Lake Tahoe? -Mental Model 058

# Fermi Problem

A question that seeks a fast, rough estimate of quantity which is either difficult or impossible to measure directly.

**Example:** Fermi problems are often known as back-of-the-envelope calculations, such as; *How many drops of water are there in Lake Tahoe? *Solving that problem requires an estimate of the volume of the drop, the volume of Lake Tahoe from its approximate dimensions and conversion of units to provide the answer.

**Wisdom:** “The first method for estimating the intelligence of a ruler is to look at the men he has around him.” ― Niccolò Machiavelli