When the Royal Family were in a hurry in the morning (which was often) the King would often help the Lightning Kid dress more than he ought to - if he was to learn to dress himself independently, having a parent do all the work was not the way forward. Similarly in the evening, tired parents, royal or not, try to expedite the bedtime routine so that the children are down sooner, and a peaceful evening can be enjoyed.
For the Lightning Kid that meant stripping him of his clothes to change into pajamas, of course. When the King realized how much he was interfering with the Prince's independence, he backed his assistance off. He was a King, not a servant, after all.
Sure enough, the Lightning Kid was often able to put on and take off many of his shirts and trousers on his own, though his methodology was unorthodox to say the least.
It was then the King observed a trick he had not seen since he had watched acrobatic dancers on the Southern Isle - the Lightning Kid could rotate his arms in a complete circle that was aligned in a vertical plane! His arms stayed parallel to the walls all the way around.
While the King marveled at the phenomenon (watching it made the Queen a little queasy), Shark Boy demanded to know why he couldn't do it, and what it meant to be "double-jointed". The King's explanation that some people were simply made differently didn't really satisfy Shark Boy, and he tried to convince himself that he was also double-jointed, and tried to fake his way through it. A powerful imagination, it turns out, was a special ability endowed upon the elder Prince, as it turns out.