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Two chemists go into a restaurant.
The first one says "I think I'll have an H2O."
The second one says "I think I'll have an H2O too" -- and he died.
Why?
The first chemist asked for H20, and that's water.
However, the second chemist asked for H2O2, which is Hydrogen peroxide.
Although not always lethal, H202 can cause serious harm.