Tippe top, also called inverting top or flip top. Known to have amused nuclear physicists, namely Niels Bohr and Wolfgang Pauli. For more details, check the top's Wikipedia page: http://en.wikipedia.org/wiki/Tippe_top.

Getting the top to invert was a little tricky. I couldn't find the right proportions for the top online, so I tweaked the parameters in the openscad script until the top started working. It can probably be improved, tippyness-wise.

Print two copies/halves with 100% solidity and glue them together.