in my aquarium Ive got a little pump for bubbling air through the water so it is substained with enough oxygen. Internaly the pump works fairly similar to old door bells, where the mains is flowing through a coil thus creating a magnetic field which moves an armature, which again presses air thorugh a hose and underneath the water's surface. (As from the German Wikipedia, https://de.wikipedia.org/wiki/Schwingankerpumpe)
Now one day I looked at my pump as I found out that its housing is very tight so that air cant come to the inside. I wrapped it up completely in plastic foil, but the air output didnt reduce. I followed the hose with my eyes, but it (obviously) didnt show any holes in it.
Still, I can use the pump (without any possible air input) to fill up a balloon.
How is that possible?