Increase in Pen Population

When the wind blows and the weather is unsuitable for any beast to be outside, it is nice to be able to loiter indoors. I was for example tidying shelves and drawers and found pens everywhere. Regularly I give away almost all my pens because experience has taught me that they multiply in their jar. I can by no means see where they come from. To the best of my knowledge I do not steal them and I never buy a pen. Test tube fertilization or pen love-life?