Usually, to get anything cold enough to make a Bose–Einstein condensate you must first trap the boson using magnets, then, by bouncing lasers off them, take all of their energy away (Laser cooling). But, this still doesn't get it cold enough. Some of the particles will still be bouncing around a lot, and only some will be lying down as needed, so the magnetic field is slowly lowered bit by bit, to let the faster bouncing particles out. This leaves the coldest and slowest atoms inside.