Chapter Five, "Diagon Alley"
"'I never know,' Harry called to Hagrid over the noise of the cart, 'what's the difference between a stalagmite and a stalactite?'
'Stalagmite's got an "m" in it,' said Hagrid."
(This is one of my favorite exchanges in this book -- Hagrid is great!)
(And it took me forEVER to get the joke about "Diagon Alley" -- yeah, I'm slow!)