Airplanes towing messages is 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.

But the physics behind a flying sign is more complicated than one would expect. Most of the planes that carry these signs are rather small, 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 plane, already in flight, picks the sign up off the ground.

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.

Bonus fact :The top of the Empire State Building was originally designed, at least superficially, to be a landing spot for dirigibles (that is, Zeppelins), giving passengers a way to enter midtown Manhattan in style. According to the New York Times, the dirigible landing area was nothing more than an excuse to make the building an extra 200 feet tall in order to make the otherwise 1,050-foot tall building clearly taller than the previously tallest building, the Chrysler Building (itself at just under 1,050 feet). No airships ever docked at the Empire State Building, as conditions were quickly deemed unsafe for such activities.

From the Archives: The Spirit of Butts Farm: The story of a transatlantic flight — of a radio-controlled model plane.

Related: A LEGO Empire State Building. Some assembly required.

Image via BabyClipart.net