Music Rocks (Or It Doesn't)

About a month or so ago my husband was dropping off the baby at his parents' house. They were watching the baby for most of the afternoon and into the evening while we were out with friends. When my husband picked up the baby, his mom mentioned that he'd cried a lot of the day, was fussy and cranky, and barely napped. 
The next day I was trying to figure out what was going on that caused him to be so cranky and not sleep. I asked my husband if Baby was asleep when he dropped him off, because he usually is. He loves to sleep in the car. My husband said, "Actually, no, because I kinda forgot he was in the car and I had my music on loud. So he was wide awake when I dropped him off". We had a good laugh about it, but I was kind of wondering how he could forget the baby was with him.
Then the other day I was out running errands by myself. At first, I was driving along in a silent car. Finally, after 10 minutes, I realized that Baby wasn't with me and I could listen to music. So I cranked it up! Then I remembered what had happened to my husband and I realized that I'm used to the baby being in the car and he isn't.