[kennu]

Brushing teeth is a good example of activity where you use almost zero cognitive effort, because the daily routine is so automated. That's why it's so easy to forget to buy more toothpaste - you just don't think about it at all. I completely get the sentinel idea as a trigger that turns your brain back on. It's probably not just the different packaging, but also the different taste, that keeps reminding about it until you buy more of the usual stuff.