These are small rare earth magnets - 0.25" 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.
Activates the reed switch (shown below) at a distance of about 3cm.
Sold in single units.