I love this one so much that I've memorized it and I quote it in my head whenever I make hot cocoa:
[Calvin is making cocoa and Hobbes is watching him]
Calvin: The secret to making great hot chocolate is to put the tiny marshmallows in first.
Hobbes: So they melt faster?
Calvin: No, so you can fit in forty or fifty of them. This way the hot chocolate just fills in the cracks.
Hobbes: I wondered why you eat it with a fork.
Calvin: Also, I don't use milk. I just heat the syrup.
And these two, which I must recite in my head when making a peanut butter sandwich:
Calvin [to his mom, who is off-panel]: How's my peanut butter sandwich coming along? You're using chunky peanut butter, right? I won't eat smooth! [next panel] And make it an open-faced sandwich, too! Don't put any jelly on it or anything! And use some normal bread! I don't like those weird grain breads! [next panel] Did you cut it diagonally? I like triangles better than rectangles, so be sure to cut it right!
[Calvin's mom sets the sandwich down in front of him]
Mom: Your majesty's sandwich.
Calvin: HEY! This is a closed-faced, horizontally cut, smooth peanut butter sandwich on weird bread with jelly! Weren't you LISTENING?
[Calvin is looking aghast into a jar of peanut butter]
Calvin: Aaugh! The peanut butter is ruined! [next panel] You're supposed to scoop one half straight down and then dig out the other side from the bottom, so part of the top remains undisturbed until the very end!
Calvin's mom: What on earth for?
Calvin: It's a ritual! You have to keep the top of the peanut butter smooth!
Calvin's mom: Maybe you should make your own sandwiches.
Calvin: If you can't control your peanut butter, you can't expect to control your life. Did you cut the bread diagonally? [I quote the first sentence of that panel ALL THE TIME.]