| The little dimple that can be found right under your nose. A great place to rest the end of your pointer finger (or a Faber #2 pencil) when thinking "hmm...".
I'm not sure of the exact "reason" for the philtrum, so I guess it's right up there with the appendix — although I've never heard of anyone having to have an emergency filtrumdectomy. Perhaps it provides a clearer path for for air flow during sniffing. I have some time booked at the local wind tunnel to test this theory — wish me luck.
I once mistakenly believed that the word was spelled "filtrum" — luckily geeklizzard enlightened me.
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