Let's say you want to know if type of soil affects how long it takes a seed to germinate. You decide to set up a controlled experiment to answer the question. You might take five identical pots, fill each with a different type of soil, plant bean seeds in each pot, place the pots in a sunny window, water them, and measure how long it takes for the seeds in each pot to sprout. This is a controlled experiment because your goal is to keep every variable constant except the type of soil you use. You control these things!