Fob watches are the key to regeneration. They serve as a storage container for regenerations and as a method to gain regenerations. They are also indestructible and can not be damaged by Earthly means.
How to obtain a Fob Watch?
A fob watch can be crafted using two different recipes.
They both bestow regenerations, they are just aesthetically different.
How to use a Fob Watch?
To get regenerations from a freshly crafted fob watch, you right-click it. This will transfer all the regenerations from the watch to you. By default, this will grant you 12 regenerations but by messing with the configs, you can make it any number you want. There is also a config for giving yourself infinite regenerations.
How to store regenerations into a Fob Watch?
By shift right-clicking the ground with the fob watch in hand, you can willingly transfer one of your regeneration back into the watch. This can be used for many things like trading with regenerations or making sure you always have backup fob watches.
How to dispose of a Fob Watch?
Fob watches are naturally indestructible. You can burn one in deepest lava of the Overworld, throw it in smallest crevices of the Nether, and crush it with quartz pillars but it will survive. The only known way to destroy the watch is to throw it in the void. If you have a Tardis, you can just dig a hole in the engine room and you have a void but in vanilla Minecraft, you have to resort to the End. Simply throw the watch over the edge and watch it die.