It is important for an experiment to be a fair test. You conduct a fair test by making sure that you change one factor at a time while keeping all other conditions the same. For example, let's imagine that we want to measure which is the fastest toy car to coast down a sloping ramp.
Adding on to what the person above me said, cigarettes can also be very addictive due to the nicotine. Once you start smoking, it can be extremely difficult to stop.
