Earlier this year, a man posted on Facebook that his mother-in-law had just laughed at him for not knowing what the tiny hole at the bottom of nail clippers is for. The post went viral. Thousands of people piled on to confess they’d never given much thought to this everyday object. Some said they assumed it was decorative. Others had theories. A few, including myself, boldly claimed they’d always known. (I said as much to my husband, who gave me…