2) Pluto's orbit crosses Neptune, but they'll never collide.
When I was a kid, I knew that for almost exactly 20 years every orbit, Pluto was closer to the Sun than Neptune. But it baffled me that they could never collide -- 248 years may seem like a long time for Pluto to go around the Sun once, but over the lifetime of the solar system it's made about 18,000,000 circuits, plenty of chances to wipe out into Neptune. So why hasn't it?
The problem here is in the way this is usually described: people say Pluto's orbit
crosses Neptune's. But the orbits do
not cross! That is, they never actually intersect. However, Pluto does get closer to the Sun than Neptune can. How can this be?
It's because Pluto's orbit is
tilted with respect to Neptune's. When you look at an image from directly "above" the solar system (I know, there's no up in space, but astronomers call
up the direction you go straight up from the Sun's north pole), it looks like the orbits of Neptune and Pluto intersect. But Pluto's orbit is tilted by about 17 degrees compared to the plane of the solar system (defined as the plane going through the Sun's equator-- it all goes back to our friendly star).
If you look at a diagram of the orbits as seen that way, Pluto's orbit never physically crosses Neptune's; it goes above it when Pluto is nearest the Sun. So if you look at things from an oblique view, you can see the two paths never meet.
As usual, things may look one way, but if you take a step back -- or to the side -- you wind up learning something.