A small subtractive synthesiser. Choose a waveform, shape its brightness with the lowpass filter, and set how each note starts and ends with the envelope sliders. Play using the on-screen keys, or the computer keys A to K (with W, E, T, Y, U for the black keys). Press Start audio first.
What to notice: lowering the cutoff makes the sound darker without changing its pitch, and the envelope changes whether a note feels like an organ, a horn or a plucked string.