Why don’t try to make a ”can-phone” (have no idea if this is the correct term)? Many have done it sometimes.
Try to experiment with it; how long thread can you use?, what difference does the thread quality make?, can you listen around corners?…
When you’re done experimenting, let the kid use it to play with, and they’ll might find more questions to ponder…
The manual (in Swedish) is found from experimentskafferiet.se, though their site seems to be down for the moment.