Short answers, pulled from the story.
The harmonica is a free reed wind instrument also known as the French harp or mouth organ. It is used worldwide in blues, American folk music, classical music, jazz, country, and rock. It is played by directing air into or out of holes along a mouthpiece, where reeds behind each hole produce the sound.
The blues harp is the most common type of harmonica, a diatonic Richter-tuned instrument with ten air passages and twenty reeds. Its nickname Mississippi saxophone comes from the cupped-hand, amplified sound pioneered by Marion "Little Walter" Jacobs.
Bending lowers a note's pitch through embouchure adjustments and gives the blues harp its wail. On unvalved diatonic harmonicas, players can also overbend, overblow, or overdraw a pair of reeds sharing a chamber. In the 1970s Howard Levy developed the overbending technique, which combined with bending let players reach the entire chromatic scale.
Christian Friedrich Ludwig Buschmann is often cited as the inventor of the harmonica in 1821, though other inventors developed similar instruments at the same time. Free-reed designs began appearing in Europe around 1820, and Richter tuning was created by Joseph Richter in 1826.
The comb is the harmonica's main body, traditionally made of wood but now also made of plastic ABS or metal, including titanium for high-end instruments. Wooden combs can absorb moisture and swell or crack, while the main advantage of a comb material is its durability.
The main types include diatonic, chromatic, tremolo, octave, orchestral, and bass versions. The chromatic uses a button-activated sliding bar to reach all flats and sharps, while the tremolo has two reeds per note tuned slightly sharp and flat for a warbling sound.
Playing the harmonica requires inhaling and exhaling strongly against resistance, which builds a strong diaphragm and deep breathing. Pulmonary specialists compare this to exercises used to rehabilitate COPD patients, and many pulmonary rehabilitation programs now incorporate the harmonica. When Ronald Reagan suffered a punctured lung in 1981, his breathing therapist was Howard McDonald of the Cambridge Harmonica Orchestra.