A doctor is operating on a patient.
He says to his assistant: “Helium please” so the assistant wheels over a tank of helium. The doctor proceeds to put a mask on the patient so he can inhale the gas, but the patient doesn’t respond to the treatment.
The doctor turns to his assistant again. “Curium please”. And the assistant produces a syringe full of the nuclear medicine. The doctor injects the patient, but again they don’t respond to the treatment.
The doctor turns again to his assistant: “Barium please?” The assistant looks surprised. “Barium, why do you need barium?”
The doctor says “Well if you can’t helium, and you can’t curium, the only thing left is to barium!”