Never a truer word spoken or typed in this case. I wear glasses and you can always bet that I will run a dirty finger across a lens leaving a nice smear. Another thought I had is, always go to the toilet before one starts working on the dirtiest job as you can guarantee once you are elbow dirt in dirt, nature calls...
Sorry, I'm repeating some of Emm's post but it still is excellent advice whether one uses a bathroom or toilet and especially if one needs to go indoors to the main house to use the little room...