Now that SeanB has given you every example of NOT what to do...
Seriously, the organic electrolytes in some of the newer cells can have a variety of different issues, in addition to being flammable.
Any Li based cell may contain small amounts of other heavy metals, that you'd rather not put into a landfill.
Why not put a little effort into this, and do what you should do with old cell phone, computer, portable tools, etc... batteries, recycle them!
http://www.call2recycle.org/By the way, the idea of taking an 18650 and puncturing it with a nail, and dropping it into water is a really bad idea. If the cell has been abused there may be metallic lithium present in the cell. When one of these is punctured, water vapor in the air could cause the lithium to ignite, resulting in a fire, and the puncture from the nail may be inadequate to vent the pressure, or may plug with carbon, resulting in an explosion. But if you'd like to see if you can lose a hand...