Solid is one of the three main states of matter, along with liquid and gas. Matter is the "stuff" of the universe, the atoms, molecules and ions that make up all physical substances. In a solid, these particles are packed closely together and are not free to move about within the substance.
The higher in the atmosphere, the less oxygen, and the more cold temperatures.
The material/moisture on the plane would freeze and damage the engine.
A mechanical wave always consists of wiggles in some kind of material stuff. The material stuff may be air, water, rock, wood, steel ... almost anything, as long as it's some material. In general, whatever the waves are moving through is called the "medium". If there isn't any, like the vacuum in space, then mechanical waves can't travel there. For example, sound can't travel in space ... not even an inch.
