Mathematics
Mathematical Induction and Its Variants: Strong and Structural Induction
Quick fact
Mathematical induction lets you prove infinitely many statements in one fell swoop: you show that if any one domino falls, the next one falls too, and then you tip the first one. This single principle is equivalent to the well-ordering property of the natural numbers.
Why this is interesting
You know that a row of dominoes will all fall if you tip the first one—but how do you prove that the entire infinite row falls without ever checking each domino?