If ritalin (methylphenidate) is a stimulant how does it calm kids with adhd?

The basics: The basic issue with ad/adhd is thought to be the inability to screen out external stimulus form reaching the thinking processes. The stimulants "turn on" a better screening center to eliminate the distracting inputs and allow a greater ability to focus and/or control activity overflow.At higher doses it often slows the kid and makes them sleepy.

ADHD: Imbalance of neurotransmitter metabolism in brain plays primary role in adhd. There is reduced activation of certain parts of brain (prefrontal-striatal-thalamocortical). Medication like Ritalin (methylphenidate) stimulates release of such neuro-transmitters in the brain to bring to a balanced state so that the person has more self control. For this it is called stimulant. It doesn't stimulate the kid.