My son used to act like he was sick. He had only been really sick a few times and to him it meant staying in bed, room-service popsicles and a movie marathon.
Last year he was doing the pretend sick thing a lot and happened to pull the "I'm sick" sad face on the morning he already had a check-up with vaccinations scheduled. He didn't know about the appointment so he was really surprised when I said "Oh no, you are sick! Get in the car we need to go to the doctor. They can give you a shot and hopefully you'll be better soon"
He doesn't pretend to be sick much anymore. He is more honest now. He'll just ask for a cozy day at home.
