This chapter investigates suffocating candles and how long a candle stays lit in jars of different sizes. The students’ task is to try to find out what the uncontrolled variable is and how to control the variable, while using their knowledge and experience to guess what the variable might be. The challenge here is to get the same result again and again. The closer the repeatability the better, but if the spread is greater than 10 percent this could be a sign that the experiment is not well controlled. In this case, it’s a good idea to search for errors.