In the words of Detroit icons Insane Clown Posse, "Fucking magnets, how do they work?". I don't know, but I do know the lid to this yellow 20 oz Yeti Rambler has them. This tumbler keeps your Faygo cold and your coffee hot. 

This cup is BPA-free, dishwasher safe, and has a No Sweat Design to make sure your hands stay dry. 6 7/8 in high; lip diameter 3 1/2 in. Fits in standard-sized cup holders.