You may have them in your house.
You may be aware of their position as a barrier between interior and exterior areas.
You may be under the influence of three glasses of cask wine and forget how to navigate around stationary objects.

-Danger-
"A Sliding glass door or Patio door, a type of sliding door in architecture and construction, is a large glass window opening in a structure that provide door access from a room to the outdoors, fresh air, and copious natural light. A sliding glass door is usually considered a single unit consisting of two panel sections, one being fixed and one a being mobile to slide open." - Wikipedia.
Please spread the word about how dangerous these things can be. Tell your friends to keep their beady little eyes open when intoxicated and fighting gravity. Inform your family on the importance of visible strip decals highlighting near-invisible glass where reflections of their soulless, ginger-bearded faces don't appear. Nobody wants a semi-broken nose ruining their planned Saturday of shame and nobody wants to have to apologise for their behaviour while sounding really, really, more-than-normally nasal due to facial trauma.
Remember to keep your eyes open.
They're everywhere...
For further info see 'Imprints Of Inebriation: Friday Nights Of Regret' and 'How I Learned To Fight Back Against Gingers By Standing My Ground' - Dora Glass.