The prank involved Johnson's Amphicar, the only civilian amphibious car every mass produced, according to the National Park Service's website for the Lyndon B. Johnson National Historical Park. 3,878 Amphicar vehicles were built in in Germany during the 1960s in four different colors. Johnson chose Lagoon Blue. On land, the vehicle looked like any ordinary automobile, and the President offered to drive guests around his ranch.
Those leisurely rides created the opportunity for a clever prank that Johnson enjoyed playing on his passengers. It involved barrelling the vehicle down a hill into a lake and exclaiming to his passengers that the brakes had malfunctioned, while they had no idea the car was designed to float in water.
Joseph A. Califano, Jr., the president's special assistant for domestic affairs at the time, recalled his experience as a victim of his boss's prank:
The President, with [White House secretary] Vicky McCammon in the seat alongside him and me in the back, was now driving around in a small blue car with the top down. We reached a steep incline at the edge of the lake and the car started rolling rapidly toward the water. The President shouted, "The brakes don't work! The brakes won't hold! We're going in! We're going under!" The car splashed into the water. I started to get out. Just then the car leveled and I realized we were in a Amphicar. The President laughed. As we putted along the lake then (and throughout the evening), he teased me. "Vicky, did you see what Joe did? He didn't give a damn about his President. He just wanted to save his own skin and get out of the car." Then he'd roar.
Secretary of Health, Education, and Welfare, John W. Gardner, one of the only Republican cabinet members during Johnson's presidency, was also reportedly a victim of this prank.
The Amphicar was known for its easy transition from land to water. "The process was simple," Rebecca Onion wrote for Slate. "One: drive the car into the water. Two: engage the two propellers, located under the rear engine compartment. Three: Steer as usual - the wheels acted as stabilizers."