Magnets produce a special field that excites material containing iron (steel is a common material containing iron). When excited, the material wants to stick to the magnet. Or - the magnet sticks to the material (ie. the fridge - good luck moving the massive fridge with the very small magnet).
For this experiment, take a magnet (preferably a free one from restaurant advertisements) and go on a hunt to find where the magnet sticks!
Or just make it stick wherever...
Aw man, that didn't work.
Make sure you record your findings! Any surprising places? The fridge is a given, but my favorite is the dishwasher. What do your findings tell you about the material of those items?
If you get a stronger magnet, you can see what the magnet picks up. Use your imagination for picking out items (think office supplies, tools, stuff with steel). If some of your selections don't work, that's part of the learning process. Make sure your kid knows that some things didn't work and they are of a different material that doesn't have magnetic properties (ie. plastics, ceramics, etc.).