The human eye does not observe everything it sees and the mind does not capture all the significant characterstics. For this reason experimentation, recreating the phenomenon and repeating it, helps to capture these characteristics.

It is necessary to extract the essential of the phenomenon being studied and design a simplified replica of it, in this way eliminating the the aspects which can hide what is essential. 
The variables which intervene in the phenomenon are modified one by one and and their effect on the phenomenon is tested. It is essential to modify one CAUSE at a time see the EFFECT which it triggers. Then it is necessary to record all the data. The variable that is modified by the scientist is calle the independent variable. The variable which changes as a consequence of having varied the independent variable is called the dependent variable.

Galileo did not begin to swing a lamp in his house, he simply took several balls and made them swing one by one tying them to a rope. He repeated the oscillation, and measured aspects of it: the time of the oscillation, the length of the rope, the mass which swings, the angle (separation from the vertical). To measure the mass, the length and the separation was easy for him, but to measure the small amount of time was very dificult in that era. Galilieo did it by counting his heart beats.  

Click on pendulum and carry out the activities proposed in the visual. Which is or are the independent variables? And the dependent ones?  

Activity 1:  The visual should not substitute for practical sessions in the laboratory. Make a pendulum in your house and try the same experiments. 

Activity 2: Using your watch find out the time which elapses between two beats of your heart. Is it always the same?