Our daughter Aisling would like to know why we have evolved two bodily systems to excrete waste products. Why do we have to both poo and wee?rn1. Strictly speaking, the question is misplaced. We do not "excrete" faeces because our bodies are long fleshy tubes which can be thought of as extremely elongated doughnuts. In a doughnut, one would not consider the hole to be "inside" the cake. Similarly, the tube from our mouth through our gut to our anus is technically "outside" our living body.
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