Additive synthesis — build sounds from harmonics and hear them
Additive synthesis builds any periodic sound by summing pure sinusoids (harmonics). A square wave contains only odd harmonics (1, 3, 5...) with amplitudes 1/n. A sawtooth contains all harmonics 1/n. Fourier's theorem guarantees any periodic waveform can be represented this way — this is the mathematical basis of all digital audio synthesis.