"So why do kids with ADHD fidget and wiggle and run and jump and bounce and scream and play so much? Kids with ADHD are understimulated, which means that their thresholds are so high, that the stimuli in their environment does not cause them to release enough neurotransmitters to fit into all the necessary receptor sites. Messages don't pass from one neuron to another as easily as they do for those of us without ADHD. Their thresholds are high. Kids with ADHD fidget and squirm in order to provide extra stimulation, which translates into more keys fitting into more locks, and they can pass messages efficiently. Ever studied something intensely and then noticed that your leg was bouncing? Same thing. You were bouncing your leg to stimulate yourself and send a sufficient number of neurotransmitters into the synapse. When kids have to stimulate themselves, it can be hard on everyone around them, since this translates into bouncing off the walls."