Anyone who has seen a water balloon full of water... has seen the veil. Or at least a decent representation of it. A thin membrane seperating two different things. Able to withstand and handle periods of elevated force or a difference in pressure... yet... unable to withstand a pinprick.

Thankfully, unlike said balloon... when the veil is pierced, it will generally heal itself. Generally. When it doesn't is when you observe portals between realms. And the flow of energy and substance goes in both directions. Each side becoming diffused and contaminated with the other sides' energy and conceptualizations. Because the overlapping region is dominated by the mixed structure.