My seven-year-old son loves astronomy-- so much so that we read space books before bed time. One of our books talks about Pluto's orbit crossing over Neptune's orbit and will be (or is) closer to the Sun than Neptune for a period of years.
I'm assuming the orbits actually never have a collision path, but what will happen when Neptune and Pluto get as close to each other as possible-- will Neptune's gravity do something to Pluto's orbit?