Note: Usually if a joke needs need be explained then it's not a good joke. However, because this joke is scientific an explanation is provided at the end.
Two chemists walk into a bar.
The first one says, “I’ll have some H2O.”
The second says, “I’ll have some water too. But why’d you order it like that? We aren’t at work.”
The first chemist excuses himself and weeps in the bathroom.
His assassination plot had failed.
Explanation: The second chemist says, “I’ll have some water too." What was supposed to happen was the second chemist saying, “I’ll have some H2O too." As in H2O2 or hydrogen peroxide.