Once when I was planning a car trip with my four-year-old daughter she was exposed to measles (I think) and I didn't want her coming down sick on the trip so I took her off to the doctor to see what could be done.
He said that it wasn't a guarantee but that a GG (gamma globulin) shot would give her greater immunity so I said OK.
I carefully explained to my daughter that the shot would hurt but that it would be over very quickly. She put on her brave face and held out her arm. She winced with the shot but smiled when it was over and turned to me. Then the nurse said, Now, the second shot goes in the other arm.
My daughter cried. I felt absolutely awful. If we had known there had to be two shots I could have helped prepare her.