A perfect gift for a thrifty mathematician.
(Make sure that he/she is not one of those topology haters)
This is a piggy bank (coin bank) in shape of Klein bottle.
You put money in a hole and it travels trough a tunnel into a belly of Klein bottle. It accepts coin sizes of euro coins (I can't say for other currency coins).
I think it looks really good when printed with transparent/ semi-transparent material and with one perimeter thick wall.
If you decide to print with only one perimeter thick wall, please check G-code in g-code viewer to make sure that wall is only one perimeter thick. If it is not only one perimeter thick, you can either choose STL file with thinner wall or you can make wider extrusion width in advanced settings.
If you want stronger and thicker walls, choose the one with 1.5mm thick wall.
Numbers in the name indicates wall thickness: "Klein_bottle_piggybank_01" means wall thickness of 0.1mm and "Klein_bottle_piggybank_02" means wall thickness of 0.2mm and so on, until the thickest one "Klein_bottle_piggybank_15" which means wall thickness of 1.5mm.
For better looking prints, you can also lower layer height towards the top of the Klein bottle.
If you have any questions or wishes for different wall thicknesses or maybe just bottom thickness, please make a comment, I will see what I can do.