Before January 1, 1913, the U.S. Post Office didn’t accept packages weighing more than 4 pounds. If you wanted to ship something heavy, you had to use expensive private express companies.

When the Parcel Post service launched in 1913, the weight limit jumped to 11 pounds (and later up to 50 pounds). Suddenly, Americans could ship all kinds of large items—like heavy tools, bricks, and, as it turned out, their own kids. Because the initial regulations didn’t explicitly state that humans couldn’t be mailed, a few enterprising parents saw a legal and incredibly cheap loophole.