In some cars, the only way to add or check transmission fluid is to remove a hard-to-reach plug and add fluid until it starts pouring out the hole. Since the hole is in the side of the transmission, there's no way to get a funnel into the hole. You have to pump the fluid into the hole.
Auto parts stores will sell you a pump, but I discovered that all I needed was a water bottle and some tubing. Drill two holes in the cap for the tubes, fill the bottle about halfway with fluid and force air into the short tube. I was worried about air leaking between the hole and the tubing, but it came out just right, nice and tight. It can still work without a perfect seal but it'll require more air.
This tubing was the perfect diameter to thread into the nozzle of the air gun on my compressor, but I think it would have worked just as well to simply blow into the short tube. (I tried it with water just now and it seemed to work fine, but watch out for fumes and don't let any fluid come back up the short tube!)
The tubing shown here is for the ice maker in my fridge.
The tubing shown here is for the ice maker in my fridge.