The idea behind this is to teach young kids the idea of creating your own compass using only water and some simple materials. This can be then further used to help explain the idea of magnetic poles and rotating electrons, which causes the needle to always point north and south. https://en.wikipedia.org/wiki/Earth%27s_magnetic_field
The little hole on the top is for a 10 mm diameter by 2 mm depth magnetic, which can purchased from Amazon or other retailers. https://www.amazon.com/Cutequeen-Ceramic-Industrial-Ferrite-Refrigerator/dp/B06XXTQQ35/ref=sr_1_4?keywords=magnets&qid=1571696207&sr=8-4&th=1
This product was created using the open source 3D CAD software FreeCAD.
At a price of $20 per kilogram, this compass only costs $0.56 to print, 100 magnets to fit in the designed hole would be $0.08 per compass, and a sewing needle is less than $1. Making the total cost of this to be less than $1.75, where a traditional single compass on amazon is $6.39.
Saving $4.50 per compass for the demenstration or learning experience.