Don't be surprised if your teacher disappears soon. The military is going to want to hush him up before the public becomes wise to their top secret weapons development.
I sometimes call myself a "professional liar to children", because a lot of what we teach children is actually wrong, but it provides a useful short-term model until we go into more detail at a later stage (such as particles -> atoms-as-marbles -> electrons in shells -> electrons in clouds).
My daughter used to believe that the sun went down because I put it in my garage for the night. (Who would tell a poor child such a thing?) It's fun for a while, but eventually I got into a little trouble with her teachers. :D
You can put the flashlight in the zip lock bag with the water, freeze it solid and use it to whack someone, hit them enough times and it might be deadly.
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You can, however, use a hanging bag of water as a crude lens to focus and concentrate the light from the torch.