Read this strip at gocomics.com.
In an unusual inversion from the norm in later strips, here it's Charlie Brown's imagination that's active and Snoopy is the realist. I can't help but think Charlie Brown realizes his little sled ride's kind of pathetic; otherwise why would he talk it up with exclamations like "Down! Down! Down!" or "Racing like the wind..."
The chain he's using to hang onto the sled is a nice touch, as is the care Schulz uses to draw the sled. It's very well-rendered!
The lead panels, which can be kept or left off of a strip at the newspaper's option, are a continual problem with Peanuts' storytelling. Schulz has to write each strip so that it works either with or without those panels, which sometimes messes with his timing. Here he presents what is probably a little too much lead-in, which slightly damages the joke.
EDIT: As Sarah Loyd rightly noticed, Snoopy is sporting a chagrimace in the next-to-last panel.
