●  Why is  （71） fun、 What delights may its practitioner expect as his reward、 First isthe sheer joy of making things. As the child delights in his mud pie, so the adult enjoys buildingthings, especially things of his own design. Second is the pleasure of making things that areuseful to other people. Third is the fascination of fashioning complex puzzle-like objects ofinterlocking moving parts  and watching them work in subtle cycles, playing out theconsequences of principles built in from the beginning. Fourth is the joy of always learning,which springs from the  （72）  nature of the task. In one way or another the problem is evernew, and its solver learns something: sometimes  73） , sometimes theoretical, and sometimesboth. Finally, there is the delight of working in such a tractable medium. The  （74） , like thepoet, works only slightly removed from pure thought-stuff. Few media of creation are soflexible, so easy to polish and rework, so readily capable of realizing grand conceptual structures.    Yet the program  （75） , unlike the poet's words, is real in the sense that it moves andworks, producing visible outputs separate from the construct itself. It prints results, drawspictures, produces sounds, moves arms. Programming then is fun because it gratifies creativelongings built deep within us and delights sensibilities we have in common with all men.