These are very small rare earth magnets - 0.125" (~3.2mm) cubed. Composed of Neodymium/Iron/Boron (NdFeB), these magnets are curiously strong and make for a great way to attach tools to a metal workbench, hold small children to a fridge, and as a trigger for reed switches. Don't let the photo fool you - these are very small magnets.
Activates the reed switch (shown below) at a distance of about 1cm.
Sold in single units.