Two scientists walk into a bar.

Two scientists walk into a bar.
The first one says "I'll have some H2O." The second one says "I'll have some H2O too."
The bartender gives them both water as serving hydrogen peroxide at a bar would be both illogical and dangerous.

This is the deconstruction of a different joke (which ends with the second scientist dying from drinking hydrogen peroxide). The original joke is a pun based on the chemical formula of hydrogen peroxide (H2O2), and the words "H2O too".

In this joke, the punchline is replaced with a statement that sounds like it came from Spock from Star Trek. The humor is the fact that the listener (after hearing H2O too) expects the rest of the joke to be a pun based on hydrogen peroxide's chemical formula, but it isn't.