Linus: But I thought little girls believed what was told them. I thought little girls were innocent and trusting. Sally: Welcome to the 20th Century.
Snoopy as a WWI flying ace, encountering the Red Baron. This was the first time that Peanuts fans got a really in-depth look at just HOW much of an incredible fantasy life Snoopy really has. Though brief (only 2 minutes long), it was by far the most epic thing 1966 audiences saw in animation up to that point. And just the fact that you can find this clip on its own on Youtube is a testament to its impact; for a moment that's almost 50 years old, that really says a lot!
Linus wants to mail his letter to the Great Pumpkin, but he can't reach the mailbox and Lucy won't help him. So he uses his security blanket as a bullwhip to open it, tosses the letter away from it, and Lucy watches speechless as the wind picks it up, loops it multiple times, and then wafts it right into the slot.