From Academic Kids
The melodica is a free-reed instrument similar to the accordion and harmonica. It has a musical keyboard on top, and is played by blowing air through a mouthpiece that fits into a hole in the side of the instrument. Pressing a key opens a hole, allowing air to flow through a reed. The keyboard is usually two or three octaves long.
Tenor melodicas are the most common type of melodica. The left hand holds a handle on the bottom, and the right hand plays the keyboard. Tenor melodicas can be played with two hands by inserting a tube into the mouthpiece hole and placing the melodica on a flat surface.
Soprano melodicas are thinner than tenors and are designed to be played with both hands at once; the left hand plays the black keys, and the right hand plays the white keys.