Johnny Said yes

Chemistry Level pending

Johnny and his father went to a hotel.

Johnny's father said to the waiter, "Give me $\ce{H_2 O}$. "

Johnny said to the waiter , "Give me $\ce{H_2 O}$ too!"

After they drank what the waiter brought, Johnny fell ill and his father didn't!

Assuming that this is a chemistry joke, what is the reason for Johnny's illness?

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