It's a simple matter of density. We can all agree that it would be harder to swim in dense mud than in water. Well, the same principle applies for the swimming pool. In the shallow end of the pool, you will find the greatest concentreation of kids, and the primary activity of little kids in any pool is urination. Since the density of urine of slightly higher than that of water, and the constant production of wee wee from the young ones will keep the concentration of urine much higher in the shallow end, it therefore stands to reason that it will be slightly more difficult to swim in the shallow end.

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