What is the most important thing that someone planning to enter mechanical engineering should know?

Do you like putting stuff together with your hands and figuring stuff out? Have you worked on bicycles or other motorized equipment, anything with bits and pieces?

The mechanical engineer has been called the general practitioner and the jack-of-all trades among engineering professions. This is because the profession requires education and skills that span a broad range of technical, social, environmental, and economic problems. In general, however, the mechanical engineer is concerned with controlling the principles of motion, energy, and force through mechanical solutions.
The diverse mechanical engineering field can be divided in a variety of ways in terms of job functions. Some of the most common functions relate to these areas of technology, but not all do. Among these fields are:
•Product Design -- developing products ranging from biomedical devices to gasoline-powered engines. A mechanical engineer designs anything that uses mechanical motion.
•Research and Development -- discovering new solutions to human needs or improving older methods.
•Manufacturing -- developing the machines that process materials into products. Designing and building machines and systems of machines that improve operating efficiency is of prime importance.
•Systems management -- overseeing operations of a large system, such as a power plant, as well as supervising the people who work there.
•Energy -- planning how energy is generated, stored, and moved. Industries that produce and deliver electrical power, such as natural gas, oil and alternative energy, employ mechanical engineers to develop more fuel-efficient cars, motors, and appliances.
•Marketing -- determining the need for a new or modified product, and calculating product availability, market size, cost structure, profitability, specifications, and distribution channels.

In most of these fields, the mechanical engineer is concerned with heat utilization or machine design--in other words, harnessing or creating energy. Heat utilization techniques are applied in boilers, air conditioners, and refrigeration units. Machine design is more focused on hardware, including automobile engines, computers, and washing machines.

Mechanical engineers are constantly being asked to make decisions. The size, shape, and material of every part of every mechanical product created must be decided by a mechanical engineer. They also have to determine the best and most efficient ways to manufacture the products. Often those decisions are made in conjunction with other types of engineers. Some of the decisions they make can mean the difference between life and death: the safety features of automobiles, for example, are the responsibility of mechanical engineers.