You must develop a hypothesis, or a possible explanation to answer your question.

A hypothesis is a "tentative explanation" for what we observe (Campbell, 1993).

Possible hypotheses for the above questions:

Chlorophyll, located in plant cells, causes grass to be green.
A virus which infects white blood cells causes AIDS.
Collagen, a connective tissue protein, denatures with age and causes the skin to wrinkle.

Key : The nature of today's research is to prove a hypothesis false. Experiments are designed to falsify the hypothesis by yielding evidence (data) to disprove it. If the evidence (data) that is gathered does support the hypothesis, the hypothesis is accepted on a trial basis only. It is never accepted as absolute truth. Future investigations by you, or other investigators, may falsify the hypothesis.

In our example:

If all chlorophyll is removed from the leaves of a test plant, and the plant remains green, then the hypothesis will be proved false by the data.

If all chlorophyll is removed from the leaves of a test plant, and the plant loses its green color, the hypothesis will be supported by the data.

Key: A hypothesis has to be testable experimentally in order to falsify or support it. Consider, for example, the question: Do excessively high temperatures cause children to misbehave?

Temperature is certainly a well-defined, measurable, and controllable factor, but misbehavior is not scientifically measurable. Thus, a scientist could not investigate this question.

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