Remember that the I in independent variable means I change so an example would be Julie sees commercials for antibacterial products that claim to kill almost all the bacteria in the area that has been treated with the product. Julie asks"How effective is antibacterial cleaner in preventing the growth of bacteria"? She sets up an experiment in order to study the effectiveness of antibacterial products. So she would need two Petri dishes with nutrient agar and she sprays one of the Petri dishes with an antibacterial kitchen spray. So she had two of the same Petri dishes but what she changed was she only sprayed one of the dishes with antibacterial spray.