Level of difficulty (1 being easiest, 10 being hardest): 4

As a kid, you learned than drinking bleach can and will kill you, but like most things, it turns out bleach is OK in moderation. According to the CDC, two drops of bleach can purify a quart or liter of water. Add a little more for dirtier water, and be sure to let the water sit for 20 to 30 minutes so the bleach can disperse appropriately.

[via cdc.gov]