There are many things that might leave you a bit puzzled when you take a flight… let’s look at a few!
You see liquid leaking from under the "belly" of the plane:
This is almost always water—condensation from the air conditioning system, just like the unit in your home or your car. Other times, it might be water coming from the potable water system vents. And no, it definitely isn't coming from someone using the bathroom! All waste from toilets and sinks goes into dedicated tanks, which are emptied when necessary by a specialized "lavatory truck."
You’re boarding via the stairs, there’s a loud roar, and you see the engine spinning!
Don't worry, it’s not running yet! The noise is coming from an onboard generator called the APU (Auxiliary Power Unit). It is essentially a small turbine engine (usually located in the tail) that produces electricity and air for the air conditioning, as well as the compressed air needed to start the main engines. That "fan" you see spinning inside the engine is just being moved by the wind, exactly like a pinwheel.
You see "smoke" coming out of vents in the wall...
It’s just humidity! Airliners must have powerful air conditioning systems to ensure effective air circulation and cabin pressurization. The air comes out of vents along the ceiling for the entire length of the plane. On hot, humid days, it’s common to see what looks like white smoke; it’s actually just the effect of the cold, dry air from the AC meeting the warm, moist air in the cabin.
We are on the ground, and there’s a strange smell in the cabin
The cabin's conditioned air comes from outside, usually through two intakes under the "belly" of the plane. On windy days while on the ground, it can happen that a bit of the engine exhaust gets pulled into the system; it’s only a temporary nuisance. Other times, if the plane has undergone "de-icing" before takeoff, you might smell the anti-freeze fluid used during the process.
DontWorryFlyHappy!