Open Borders

I have left my screen door propped open this afternoon, so that the cats can have free access in and out...

Unfortunately, at a certain point I noticed myriads of flies buzzing around, and even some mosquitoes and a few wasps!!

"Hmm... open borders, " I thought.

I built a wall by shutting the screen door all the way!  No more flies, or stinging horrors in my house!!

How simple is this concept?  Open borders = no safe Nation!