Clue: A little radioactive matter in contact with the molecular constituents of water: whoops! (2-2)
Answer: UH-OH
Explanation: Definition: whoops!
U (Uranium, which is radioactive to some degree in all its forms) + H O H (components of water).I am no chemist, but I believe HOH is a better description of the molecular structure of water than the more familiar H2O.