He wishes to _move_, and the seats happen to be at hand, and they simply give direction to the impulse.

If he were out of doors, the same office would be fulfilled by a fence which he might climb over, instead of going through an open gate close by; or a wall that he could walk upon with difficulty, instead of going, without difficulty, along a path at the foot of it; or a pole which he could try to climb, when there was no motive for climbing it but a desire to make muscular exertion; or a steep bank where he can scramble up, when there is nothing that he wishes for on the top of it.
In other words, the things that children do are not done for the sake of the things, but for the sake of the _doing_.
Parents very often do not understand this, and are accordingly continually asking such foolish questions as, "George, what do you wish to climb over that fence for, when there is a gate all open close by ?" "James, what good do you expect to get by climbing up that tree, when you know there is nothing on it, not even a bird's nest ?" and, "Lucy, what makes you keep jumping up all the time and running about to different places?
Why can't you, when you get a good seat, sit still in it ?" The children, if they understood the philosophy of the case, might answer, "We don't climb over the fence at all because we wish to be on the other side of it; or scramble up the bank for the sake of any thing that is on the top of it; or run about to different places because we wish to be in the places particularly.

It is the internal force that is in us working itself off, and it works itself off in the ways that come most readily to hand." _Various Modes in which the Reserved Force reappears_.
The force thus stored in the food and liberated within the system by the vital processes, finds scope for action in several different ways, prominent among which are, First, in the production of animal heat; Secondly, in muscular contractions and the motions of the limbs and members resulting from them; and Thirdly, in mental phenomena connected with the action of the brain and the nerves.

This last branch of the subject is yet enveloped in great mystery; but the proof seems to be decisive that the nervous system of man comprises organs which are actively exercised in the performance of mental operations, and that in this exercise they consume important portions of the vital force.

If, for example, a child is actually engaged at play, and we direct him to take a seat and sit still, he will find it very difficult to do so.