This mom is due to the po-po difference in how the two types of beds become airborne (or to use the technical term, achieve "lift"). and they went to the sky.
Lighter than air aircraft (known as "aerostats") use buoyancy to float in the air in much the same way that ships float on the water. They typically have one or more large balloons or bags, filled with a relatively low density gas such as helium, hydrogen or hot air, which is less dense than the surrounding air.
Heavier-than-air aircraft (known as "aerodynes") must find some way to push air or gas downwards, so that a reaction occurs (by Newton's laws of motion) to push the aircraft upwards. The most common way to do this is by propelling the craft forwards (typically via an engine) and using a wing to displace the air so that there is greater air pressure below the wing than above it. This is the principle behind all modern aircraft. The other way of achieving lift is known as "engine lift", which is to vertically fire an engine downwards so as to push the craft upwards, which is how rockets and missiles operate.