Today a friend's six year old sister asked me the question "why don't people on the other side of the earth fall off?". I tried to explain that the Earth is a huge sphere and there's a special force called "gravity" that tries to attract everything to the center of the Earth, but she doesn't seem to understand it. I also made some attempts using a globe, saying that "Up" and "Down" are all local perspective and people on the other side of the Earth feel they're on the top, but she still doesn't get it.

How can I explain the concept of gravity to a six year old in a simple and meaningful way?

Having my own 6-year-old and having successfully explained this, here's my advice from experience:


Don't try to explain gravity as a mysterious force. It doesn't make sense to most adults (sad, but true! talk to non-physicists about it and you'll see), it won't make sense to a 6yo.

The reason this won't work is that it requires inference from general principles to specific applications, plus it requires advanced abstract thinking to even grasp the concept of invisible forces. Those are not skills a 6-year-old has at their fingertips. Most things they're figuring out right now is piecemeal and they won't start fitting their experiences to best-fit conscious models of reality for a few years yet.
Do exploit 6-year-old's tendency to take descriptions of actions-that-happen at face value as simple piecemeal facts.


  Stuff pulls other stuff to itself. When you have a lot of stuff, it pulls other things a lot. The bigger things pull the smaller things to them.


Them having previously understood the shape of the solar system and a loose grasp of the fact of orbits (not how they work—that's a different piece—just that planets and moons move in "circular" tracks around heavier things like the Sun and Earth) may be useful before embarking on these parts of the conversation. I'm not sure, but that was a thing my 6yo already had started to grasp at this point.

These conversations were also mixed in with our conversations about how Earth formed from debris, and how the pull was involved in making that happen, and how it made the pull more and more. So, I can't really separate out that background; it may also help/be necessary.
Don't try to correct a 6-year-old's confusion about up and down being relative, but use it instead.


  There's a lot of Earth under us, and it pulls us down when we jump. If we jumped off the side, it would pull us back sideways. If we fell off the bottom, it would pull us back up.


You can follow this up later with a Socratic dialogue about the relative nature of up and down, but don't muddy the waters with that immediately. That won't have any purchase until they accept the fact that Earth will pull you "back up" if you fall off.
Build it up over a series of conversations. They won't get it the first time, or the tenth, but pieces of it will stick.
Don't try to instill a grasp of the overall working model. If you can successfully give them some single, disconnected facts that they actually believe, putting them together will happen as they age and mature and get more exposure to this stuff.


All this is assuming a decently smart but not prodigious child, of course. (A 6-year-old prodigy can probably grasp a lay adult's model of gravity, but if that's who you're dealing with then you don't need to adjust your teaching.)

For some more context, this was also after my child's class started experimenting with magnets at school. I was inspired to attempt to explain gravity when my kid told me that trees didn't float off into space because the Earth was a giant magnet. (True! But not why trees don't float away.) Comparing gravity and magnetism might help, to give them an example of invisible pull that they can feel, but it might just confuse the subject a lot too since I had a lot of work (over multiple conversations) to convince my own that trees aren't sticking to the ground because of magnetism, even if the Earth is a giant magnet.

And, a final piece of advice that's incidental, but can help:


Once you've had a few of these conversations, play Kerbal Space Program while they watch. (Again, this comes from experience. My kid loves to watch KSP.) Seeing a practical example of gravity at work in it natural environment will go a long way to cementing the previous conversations. It may sound like a sign-off joke, but seeing a system moving and being manipulated makes a huge difference to a young child's comprehension, because it is no longer abstract or requires building mental abstractions to grasp, like showing them a globe does.

