WHY

Why is it that most adults know how to count to fifteen, but seem to lose this ability when standing in the EXPRESS line at the grocery store. The bright light says 15 items or less, not 16, or you bought 3 cans of green beans so they only count as one. I usually stand behide them and really want to ask them if they know how to count. Do you say something to them? If you do please let me know what you say.. (If it may cause any kind of inflicted pain to me after saying it, I would love to hear it, I just may not use it).