When air is heated in a particular location, it expands and becomes less dense. This generates an upward (buoyant) force on the air, as it is lighter than an equal volume of unheated air.

"Where would the scale tip? On the left is a steel ball, on the right a ping-pong ball," Reddit user Veiy wrote, posing the conundrum. "I think the scale would raise to the right since the [buoyancy] ...

There are two forces acting on the life jacket - its weight ... Things that float are buoyant. If the weight is greater than the upthrust, it sinks.