My first two guesses about the meaning of National Mole Day were wrong. I initially thought it might have to do with the little burrowing animals, which admittedly would be a kind of odd thing to celebrate. Then I thought it might be the delicious Mexican sauce made from chiles and chocolate. But no, today’s a day for chemistry nerds. A mole is a unit of measurement based on Avogadro’s number, approximately 6.02 x 1023. That number of units (typically atoms or molecules) of any substance is known as a mole. And thus, on October 23 (10-23 in North America), from 6:02 A.M. to 6:02 P.M., we observe National Mole Day, whose purpose is to promote interest in chemistry. Go moles!