An intuitive explanation of epsilon delta proofs
First year calculus students often struggle with the level of formalism involved and don't have a good intuitive understanding. It's also inevitably on the first midterm. This page first explains some of the intuitive reasoning, and then gives some examples of proofs.

Probability and Statistics

The base rate fallacy
When a medical test comes back positive, how likely are you to have the disease? If it's rare, it's probably less likely than you think, even for a very accurate test.