What did the waitress say to the neutron when he tried to pay for his meal? “For you, no charge.”

Can you explain the punchline?

Nstrumental music that supports the main part(s) chamber music 2. a large group of instruments usually including woodwinds, brass, and percussion band 3. music written for small ensembles for an intimate setting accompaniment 4. a person who leads an ensemble conductor 5. an electronic keyboard capable of multiple instrument sounds