Atomix is an educational game designed for GNOME in which one has to build molecules, ranging from simple inorganic ones to some extremely complex organic ones, out of separate atoms.
The first levels can be rather easy, but added complexity always comes with the next level, so that several minutes (or much longer than that) can be spent on each single level.
aptitude install atomix
You can move around isolated atoms which are laying around among walls and other obstacles on the playfield.
Once you push an atom in a certain direction, it will move until it hits something that makes it stop, which could be a wall or another atom. This behaviour is exactly what makes playing it not trivial and, therefore, fun, as you need to think quite a lot about how to have your molecule organized.
The faster you are in bringing all atoms together and finally building a complete molecule out of all single atoms, the more points are awarded to you.