Q: What's the difference between roast beef and pea soup?

Q: What's the difference between roast beef and pea soup?
A: Anyone can roast beef.

This is bathroom humor. The implied punchline to this joke is "but not anyone can pee soup". The way that we get to that punchline is by noticing that roast (which was an adjective in the original phrase) was turned into a verb for the "punchline". Turning pea into a verb makes it pee (which means to urinate), and you can see that since that talks about the taboo act of urinating, it is bathroom humor.

Actually, I just realized that this joke is also a double pun, since roast has two meanings, and so does pea/pee.

Finally, this joke has a punchline that is not really the punchline of the joke - sort of like the denial joke.