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Put it this way:

You spend around 10-12 years going to preparatory school (elementary, junior high, and senior high) and then you wanted to be a doctor. So you take another ten years of studying. Then, after you graduated, you did not do anything to practice your knowledge because you are lazy.

You, as a lazy young man, will become foul as a smelling meat. As you don't put into practice the things you have learned. A man who rests in theories alone, is not a man deserving of honor. True knowledge shines bright when it is put into application.

Going back to the analogy of the doctor, you will realize that the more you slack around, not doing things, the more your knowledge becomes outdated, and this prevents you for practicing your knowledge even further.

It's like when you leave water stagnant, it attracts mosquitoes. 
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