A Big Glass of Water
A man goes to the doctor and tells him that he hasn't been feeling well. The doctor examines him, leaves the room and comes back with three different bottles of pills. The doctor says:

"Take the green pill with a big glass of water when you get up. Take the blue pill with a big glass of water after lunch. Then just before going to bed, take the red pill with another big glass of water."

Startled to be put on so much medicine the man stammers:

"Jeez doc, exactly what's my problem?"

Doctor says,

"You're not drinking enough water."

An invisible man is here to see you
Nurse: Doctor, there is an invisible man in your waiting room.

Doctor: Tell him I can't see him now. Next.