Physics question (density, buoyancy)

Question

In order to make air float, which of the following needs to be done?

Does it’s density need to be made smaller, does it need to be heated, does its volume need to be increased, or does any of the above work?

Answer 1

Just think of a hot-air balloon. How do they rise? They heat the air inside the balloon. BY heating, it gets hotter. Duh! Gets less dense and gains volume. So it displaces the colder air surrounding it and voila - it rises.

Exact same principle applies to things that float. They displace their weight in the surrounding medium - water or air. Look at flames too. Why do they always rise and never point down?