A real pea souper

Conversation at work yesterday (I'm the one trying to tell the joke):

"I finally managed to get some pea soup at lunchtime. I've been after some for ages."
"Ah! What's the difference between roast beef and pea soup?"
"What do you mean? They're totally different?"
"It's an old joke: What's the difference between roast beef and pea soup?"
"I don't know."
"Anyone can roast beef."
"Actually, that's not true; some people find it very difficult."
"It's a JOKE! Anyone can roast beef!"
"…?"
"You don't get it, do you?"
"No."
"Think about it."
"…?"
"Question: What's the difference between roast beef and pea soup? Answer: Anyone can roast beef—but…"
"…?"
"…not many people can…?"
"…roast soup!"
"Are you doing this deliberately?"
"Roast peas?"
"PEE SOUP! IT'S PEE SOUP! ANYONE CAN ROAST BEEF, BUT NOT MANY PEOPLE CAN PEE SOUP!"
"…Oh, I get it! That's quite good!"
"I used to think so."