Pippi - on observing the full moon: "How do we know that the moon is round and not shaped like a potato chip or something?"
Me: "Observation. We can see it. It's round."
Pippi: "But what if it's not?"
Me: "But it is. The first step in scientific discovery is observation. You look at something and see what properties it has."
Pippi: "But in philosophy they say your eyes can deceive you."
Right. So maybe the moon is just a big potato chip.