[da5is]

These cubes actually come completely apart to replace the springs. You could probably put a good 4-5 micro-SD cards in the middle plastic ball, but it's not exactly a quick operation. But, yeah, quite a bit of empty space in them.

[tamana]

Springs in a Rubik's cube?

[da5is]

Yes, they provide the tension when you rotate the cube. The speed cubes come with different springs that can make it looser or tighter - in the center is this white ball that houses the springs where you screw in the center tiles. If it wasn't such a pain to open back up, I'd take some pics of that part of it. You can see the white ball on the official site here: https://rubiks.com/store/product/rubiks-speed-cube (disclaimer - the original article is mine)