Resistors are commonly used to reduce the amount of current flowing from the source to the destination. To draw up an analogy, it is like squeezing a water supply tube at a particular point thus reducing the flow of water. Similarly, by placing a resistor across a piece of wire, the flow of current gets reduced.

The rule of thumb is to apply a resistor of an appropriate value, in series just before a delicate electronic component that cannot withstand too much current, for example an LED. We will look at an example of this fundamental concept in the following hands-on activity for blinking the three external LEDs in a cyclic fashion.