This plan of incidental drill is in harmony with the principle of interest previously explained. There are several things, however, that must be considered. The proper procedure would seem to be to look forward and find out in what directions the child will need to acquire skill and then to help him acquire it in the most economical way and at the proper time. Nature has so made us that we like to do a new trick. When we have taught a child how to add and subtract, he likes to perform these operations because the operations themselves give pleasure. Therefore much repetition can be allowed and much skill acquired by a direct approach to the practice. When interest drags, incidental drill can be fallen back upon to help out the interest. Children should be taught that certain things must be done, certain skill must be acquired. They should accept some things on the authority of elders. They should be taught to apply themselves and to give their whole attention to a thing that must be done. A desire for efficiency can be developed in them. The spirit of competition can sometimes be effectively used to add interest to drill. Of course, interest and attention there must be, and if it cannot be secured in one way, it must be in another. Experiments have abundantly shown the value of formal drill, that is to say, drill for drill's sake. If an arithmetic class is divided, one half being given a few minutes' drill on the fundamental operations each day but otherwise doing exactly the same work as the other half of the class, the half receiving the drill acquires much more skill in the fundamental operations and, besides, is better at reasoning out problems than the half that had no drill. The explanation of the latter fact is doubtless that the pupils receiving the drill acquire such efficiency in the fundamental operations that these cause no trouble, leaving all the energies of the pupils for reasoning out the problems.