David, in the middle of a discussion of astronomy: "And where do people come from?" Answering himself, "Oh, I know! They come from God!"
Alan: "No, they come from monkeys!"
David: "Oh, that's right! Because we are mammals! That's right!"
A little later.
David: "You remember, Mommy, you said that Pluto was no longer a planet." [That was back when Pluto got demoted.]
Doug, surprised: "That's right! Good memory, David! And do you know why some people say that Pluto is not a planet?"
David and Alan, loudly and in unison: "Because it's too small!"
Also, David wants to be a scientist who does lots of experiments when he grows up. Alan wants to be an inventor who does experiments, and goes to Mars, and creates Pokemons. Veering off on the topic of Mars, Doug later told the boys about the Mars robots, and idly asked what they might be powered with. Alan said, "Electricity," and David said, "Sunlight!".
Together, they will rule the world.