The dog's face after the second round of applause: "yes thank you, I am good boy."
Funny because in the 17th thru mid 19th century people were disrespectful during performances - it was a time to eat, drink, joke, and be merry. In a classical sonata the optional introductory passage is intended to give the audience time to settle down and pay attention before the first theme happens.
Think about it like this: in 150 years, people could be attending punk rock performances with silent "etiquette" - no interruptions, no talking, don't clap here but clap there etc
Source: B.M.
Edit: also worth mentioning clapping after a movement is a big no no. Clap in between pieces, not movements. My professor performed at Carnegie Hall though and the audience did clap in between movements, rookies. Etiquette is less strict when orchestras are dying just to fill seats