You and Leo were sitting on the beach. He was tinkering and telling you about how an airplane is able to sustain flight.

“An aircraft in straight and level flight is acted upon by four forces: lift, gravity, thrust and drag. The opposing forces balance each other: Lift equals gravity, and thrust equals drag. The thrust is the force that moves an airplane forward through the air. Thrust is created by a propeller or a jet engine…”

He makes the motions with his hands and is smiling widely.