My son has lied about being injured so convincingly I took him to the ER

Like the title says, my 3,5 year old son has lied himself into the ER. He's got a history of little lies (little sister broke it!) but nothing unusual for a young child, so this is a shock. He also really likes going to the doctor.Yesterday he just fell. His knee gave way beneath him and he fell down. He cried about pain at the back of his knee and refused to put any weight on it. He continued to cry loudly about pain and when I tried to get him to walk his knee didn't support him at all and he fell again, hard. I lifted him into the car, buckled his sister into her seat, and off we went to the hospital. He cried the entire ride there (15 minutes). Once there I lifted him into the stroller, took his sister in my arms and walked in. And he JUMPED OUT OF THE STROLLER TO GO PLAY WITH ANOTHER CHILD THERE. I was baffled. There was obviously nothing wrong with him.I apologized to the nurses, put my daughter in the stroller, took my son's hand and started walking out again. And he fell to the ground again, again crying so very convincingly about pain in his knee. But he could run and jump and play, he was really faking it. I told him off, and he walked to the car with me. Once in the car he had a serious temper tantrum, something he hasn't had in over a year. He was screeching, rhythmically hitting his head, yelling at the top of his lungs that he needed to see a doctor, and then just unintelligibly screaming for the entire ride.When we got back home, he AGAIN faked not being able to walk. I told him I wasn't going to carry him, and he ran to the front door. The rest of the evening he was contrary and angry. He doesn't seem to understand he did anything wrong. I tried explaining it to him, but he just says "yes!" when asked if he'll do it again. He doesn't answer to "why" yet, so I can't even try that. My usual parenting sites say that he's too young to understand lying and that punishing him wouldn't do anything. But he got to the freaking ER with his lies! This isn't innocent anymore, and it's hard to believe he doesn't understand but can make such elaborate schemes. This morning he did it AGAIN, so he hasn't learned anything. There really is nothing wrong with his knee.What do I do? via /r/Parenting https://ift.tt/2z4Um7r