[throwfermat]

Here is a nice one I read sometime back.

Create a video of your friend walking 3 metres. Now play the video 4 times. Your friend walks 12 metres in the video. Play the video in reverse 4 times. Your friend walks 12 metres backwards in the video.

Create another video of your friend walking backwards 3 metres. Now play the video 4 times. Your friend walks 12 metres backwards in the video. Play the video in reverse 4 times. Your friend walks 12 metres forwards in the video.

[dorchadas]

As a math teacher, I've found the best luck with teaching negatives by putting them in terms of direction. When kids start seeing "right" as "positive" and "left" as "negative", it makes a bit more sense to them. Then couple in positive/negative with forward/backward and it just generally clicks.

It also even helps explain division when you talk about it in terms of direction and how many times you have to move to get to 0. It also helps them understand why you can't divide by 0. e.g. 3/0 would be explained like "Starting at 3, how many times can you move 0 to get to 0," They can clearly see it's impossible, and it helps give them at least some basic intuition into it.

I've also found this extends to at least some properties of complex numbers as well, as you can easily extend from the number line to a coordinate plane.