Needle

bring it near something metal.

a brass compass needle over an ink chart that swings toward metal and shows the gold under its tip

a compass needle lies along the magnetic field it sits in. Earth's is about 45 microtesla; iron and magnets nearby add their own, which falls off with the cube of the distance, so a few centimetres from a laptop or a desk leg the field doubles and the needle turns to it.

Concepts: magnetism, fields, maps.

An interactive you play with one thumb, on Wundr.