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.]