To be read in the voice of Terry Thomas:
When you are going about your business on the old toilet and you are distracted mid-stool, causing widespread smearage. And after ten wipes with no clear paper, you lose concentration and brush the back of your thumb within the danger zone. The tentative bringing of the thumb towards the nose in a 'did I or didn't I' way is known in medical terms as the Juan Kerr conundrum...quite facinating....