I always thought I was a bit of a weird kid: while I had fun taking part in group activities and team sports with other children, when it came to playing  with toys I always preferred to play with my Micronauts or (twelve-inch) G.I. Joe figurines by myself.  It always felt a bit selfish to me, but it wasn't until I had kids of my own that I discovered why.  When I'd drive one of my kids to school alongside a few of their classmates in our minivan, my child would listen to music the entire time through their headphones, ignoring the conversations going on around them.  At the time I thought it a little rude, but then I realized that-- as an introvert-- my child needed the solitude of listening to music during that ride to school to build up their stores of energy for a day of forced social interaction that they could not not participate in.  When I first got into selling, I didn't know an introvert from an elbow, and just assumed that making calls for hours on end, or giving presentations to prospective customers, was naturally exhausting.