Pictured above is an airplane towing a message. It’s a curious but probably effective form of advertising: a plane flies over a location with good views, lots of people, and few other distractions (for example, a beach area in the summer), sharing the message of whoever pays its way. And they’re just unusual enough where, if you see one, you’ll probably tell whomever you’re with to look up.

And if you’re the plane’s pilot, it’s simple, right? Just attach a sign to the tail and take off!

Actually, no. The physics behind a flying sign is more complicated than one would expect.

Most of the planes that carry these signs are rather small — it wouldn’t make sense to waste an entire jumbo jet on a “buy one, get one free” ad for the local Red Lobster — and attempting to take off with the sign already in tow would throw off the plane’s center of gravity, preventing it from getting airborne. Carrying the furled-up sign in the cockpit or (typically tiny) payload runs the very real risk of getting it tangled up with the aircraft itself, resulting in disaster. In short, the plane can’t take off with the sign already attached.

Instead, the plan takes off with the sign lying on the ground. And then the plane, already in flight, picks the sign up.

As seen in this video (but you will have to look closely), the plane flies toward a pair of uprights, dangling a hook about 25 feet behind. The sign is attached to a big loop, which itself lays across a pair of uprights. When the plane flies over the uprights, the hook grabs onto the loop, lifting it — and the sign — skyward.  In order to pull off the maneuver, the plane needs to get rather close to the ground — sometimes, no more than 30 or 40 feet above the surface.  While the whole process is risky (and there have been terrible accidents), it really is the only option available.  (For an excellent essay on the entire process, click here.)

How did pilots come up with this? Advertising may not have been the impetus — perhaps, mail was.  In the 1940s, U.S. Airways used a similar technique to pick up mail from isolated areas across the country. Instead of picking up signs, though, the planes would try and grab a rubber container, holding letters and other parcels, hanging from uprights just a few feet off the ground.