How Did They Fly?
The balloons needed to be inexpensive but sturdy. They were made out of a tissue-like substance known as mulberry paper and sealed with an adhesive known as paper flour. They were then filled with hydrogen and equipped with an altimeter (a device to measure altitude).
The balloon bomb had to travel in the exact altitude range of 30,000 to 38,000 feet to reach the United States successfully. If the balloon flew too low, the altimeter would alert the balloon to drop sand bags. If it flew too high, the altimeter would release some hydrogen.