Matt,

Level can mean several things; level with the horizon generally means that the wings are level, or in other words, not banked or in a turn. level can also mean holding a constant altitude.

The simple answer is that the pilot uses instruments and follows the information on the instruments to keep the airplane level. Often, the autopilot is used. The instruments and the autopilot get their information from several sources which use air pressure and other inputs (such as speed and temperature information) to produce an accurate picture of the world outside the cockpit.

Gyroscopes, some similar to a child's toy gyroscope and others fairly complex devices that use lasers and other internal devices to sense very small changes in inertia and acceleration, provide information that tells instruments, autopilots, navigation computers, and the pilot exactly what the airplane is doing. This information is converted into displays that show a picture of the sky and the ground, and the horizon, and allow the pilot to operate the airplane much like a video game. The pilot also looks out the window, when able (when not in the clouds or over the ocean at night) to get a clear picture of what the airplane is doing, and what he must do.

Height above the ground is determined by an altimeter, which uses various information to determine an altitude based on air pressure. The higher one goes, the less air pressure there is. By using a precise instrument to measure the pressure and adjust it for things like temperature, a fairly accurate determination can be made for altitude. the altimeters are adjustable for changes in local air pressure (the result of fronts, highs, lows, and other weather features that you see on the evening news and weather channel). By using the same reference for all aircraft operating in a given area, all the airplanes can accurately be separated in altitude.

During cruise flight and most flight operations above about 27,000,' the autopilot is used for nearly everything. It's also used for arrivals and approaches much of the time, due to it's precision and ability to reduce cockpit workload. The autopilot is called by different names, generically the automated or aircraft flight control system; it's simply another means for the pilot to operate the aircraft. The pilot might have direct input on the flight controls, or may tell the autopilot what altitude, heading, course, rate of climb or descent, airspeed, etc, to fly, and control the airplane that way. Think of it as a little like cruise control in your car (the airplane features auto throttles, which work work in a similar fashion), except with functions for not just the throttle, but the steering wheel, too.

Before takeoff, a lot of preparation takes place. Altimeters are set, and the airplane is very thoroughly inspected ("preflighted"). Calculations are made regarding the weight of the airplane, passengers, cargo, fuel, etc, and a balance calculation must also be made to ensure that the airplane will be controllable and safe at all times. Fuel is calculated for weight and performance, and to ensure that not just enough is available for the trip, but emergencies, diversions, and trips to alternate airports. Navigation computers are programmed, and the plan for the flight ("flight plan") is thoroughly reviewed for errors. The flight is a series of very short legs between waypoints, which can number in the hundreds on some long flights. It works the same regardless of whether flying between two points in one state or country, or long oceanic and international flights. Additional calculations are made to cover problems that might occur between airports, such as diversions while crossing the ocean, and problems that might occur such as pressurization loss, fires, etc.

Takeoffs are flown by hand, as is the initial climb. I prefer to hand fly until a transition point which normally occurs between 27,000' and 29,000. At that point the autopilot is engaged (often much earlier than that) and stays engaged until the descent or arrival or approach to the destination airport.

The pilot uses a primary instrument that's sometimes called a primary display, artificial horizon, attitude display, or other names, which provides a basic picture of the world around the aircraft. It is divided in half; the upper being the sky, the lower being the ground, and a line through that display or instrument represents the horizon. Lines show degrees above and below the horizon, as well as degrees of bank, so that one can pitch up ten degrees, for example, or pitch down. In a bank, the instrument makes it easy to hold a constant angle (used when turning to a new heading or direction); typically most turns are twenty five to thirty degrees; what you're used to experiencing on an airliner most of the time, and less for small turns. Often today all the information comes together on one display, so that the pilot sees attitude, heading, altitude, and other information all in one place.

Backup instruments provide extra altitude, heading, and other details to the pilot if the primary information fails or loses power, and those backup instruments are often protected with their own emergency sources of power and lighting in the event of a total power failure.

The same references and skills that are involved in piloting a 747 at high altitudes over the ocean on an international trip are basic to flying a Cessna around any airport. Instruments are available that tell the pilot when the airplane is beginning to climb or descend, and the pilot or autopilot is constantly making small corrections to hold a particular altitude, or heading, or course. A course is simply a route over the ground, and today it's given as GPS information from satellites, as well as other information from onboard computers, charts, and so forth.

The short answer to your son's question is that much of what's done in flight is done by reference to instruments, as well as looking out the window. The instruments help determine height above sea level, which is how altitude is most often referenced, and local adjustments to altimeter equipment ensure that all aircraft are flying "on the same page." By carefully monitoring the instruments and the changes that take place constantly in flight, very small corrections continue to be applied to keep the airplane where it's supposed to be.