In the text find the words which mean:
1 one part of a large subject of study or knowledge
2 something that must happen first before something else can happen
3 extremely important or necessary in order to do something successfully
4 leaving out or not including
5 to try and find out the truth about something
6 to watch something or someone carefully
7 correct or normal way of doing something
8 asking someone to imagine that a particular situation existed

Whatever branch a scientist is interested in, he observes things and asks questions
about them. What is that structure for? How does it work? Why is the animal behaving
like this? Why are weeds growing here but not there?
Testing hypotheses
Suppose we want to know why a certain type of plant grows well in place A but badly in
place B. The first thing to do is to put forward a possible reason. Scientists call this a
hypothesis. The next step is to test this hypothesis to find out if it is true or not.
This is done by carrying out an experiment.
Scientists often investigate things by first thinking of a hypothesis, and then testing
it by observing things or by doing experiments. This procedure is sometimes called the
scientific method.
Doing experiments
Suppose we want to test the hypothesis that light is needed for the leaves of a young
plant to become green.
We obtain a Plant and put it in the dark. If the green colour fails to develop, we will
conclude that light is needed for it. However there is something more that we must
do: we must obtain a second plant and put it in the light. We need
this second plant in order to provide a standard with which to compare the first plant.
The second plant is called the control. In carrying out experiments it is essential that
the two plants should be kept in exactly the same conditions, except for the light they
receive. To put it in a general way: we keep all the variables constant except for the
one whose effect we want to investigate.
An experiment of this kind, in which the experimenter controls the conditions, is called
a controlled experiment.

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