The number of pens in backpack always tends to two.
Proof:
You start with one pen. Everything seems OK, but then you misplace it. So, you have zero pens. Then you take another pen to replace it. So, you back to one pen. After a while, you suddenly find the misplaced pen and you now have two pens. You think it is OK to carry two pens, in case one runs out then you have another one.