I know congruent means "the same" but I lose points explaining what similar adds.
Accepted answer
Think in terms of what transformations get you from one to the other.
- **Congruent** — reachable by *rigid motions* only (translation, rotation, reflection). Same shape **and** same size. Corresponding angles equal, corresponding sides equal.
- **Similar** — reachable by rigid motions **plus a dilation** (scaling). Same shape, possibly different size. Corresponding angles equal, corresponding sides in a constant **ratio**.
So congruence is the special case of similarity where the scale factor is 1. Every pair of congruent figures is similar; the reverse fails.
One consequence worth memorizing: if the scale factor is `k`, then lengths scale by `k`, **areas by `k²`**, and volumes by `k³`. Doubling a triangle's sides quadruples its area — a very common test question.