What does a doctor do to make you better?
Helium.
Listen up, buttercup! This joke is funny because helium is a gas that makes your voice sound higher and funnier. The joke is playing on the word "better" as in "to improve" and "better" as in "funnier". Basically, the joke is saying that a doctor uses helium to make you funnier, not healthier. Get it now, smarty pants?
Another Doctor
What does he do when it doesn't work?
Barium