I'm now sat next to a nasty piece of lithium. I'll pour water in it, will it explode? I bloody well hope not,
Did you take a piece of pure Lithium or a Lithium Battery ?
If it was the former: Be careful !!! That's gonna be some serious fireworks.
If it was the latter not too much should happen because Lithium Batteries contain only a very limited amount of Lithium itself. A 60g (2oz) Lithium Battery contains roughly 0,6g (0,02oz) Lithium i.e. very roughly only 1% of its weight is Lithium.
As I already wrote in an earlier post: The whole process in the Batteries requires significant (electric) energy in the cells to be launched. Take the electric energy out of the cells and you won't achieve a thermal runaway.
Yes they are flammable because they contain flammable substances (mainly alcohol). But self ignition only works with sufficient charge of the batteries. Without that you will have to throw them into an open fire and have to wait until the pressure breaks the can (Li-Ion).
Unfortunately the cells don't like being too empty (i.e. below a certain voltage). They will loose capacity or even fail completely (no harm just ending up as a paper weight). Therefore the manufacturers charge them too much for safe transport because otherwise the quote of defective batteries would rise.
(For LiPo the ideal range would be 3,3 - 3,5 V. Below 3 V the cell will suffer, above 3,6 V the energy starts being theoretically suffciient for self ignition, Li-Ion at a bit lower Voltage)
Edit:
Brief explanation how a thermal runaway in these cells works:
The cells contain a high number of thin layers of Copper / Aluminum foils. These layers are separated by a separator which is a sort of alcohol soaked plastic foil.
If at one place the separator is damaged, a short circuit occurs. This creates heat. The heat will melt the plastic foil further and also melt the next plastic foil separating the next metal foil coils/sheets. Thus creating a further short circuit. This continues through the whole battery. The temperature in the can will increase. At a certain point the can will break. Now the hot evaporated alcohol getting in contact with oxygen is ignited by glowing metal resulting from the short circuits and creates the flames you see in vids of this.