cause of eye-drop

apart from genes etc, what can cause eyedrop esp tank cover?
if i have a glass cover than the aro always tend to look up as its clear, not much light glare but if its all blocked hood (those black ones) will the aro be forced to look down?? will it cause eye-drop??

[Shawnsp]

One other factor is fats that is accumaltive over the eyes can cause it to drop.