Trouble identifying a control. Please HELP
 

We are doing a experiment involving bacteria growth in milk left out at room temperature. Hypothesis: Longer milk left out-more bacteria present. We will test milk using methylene blue test. Our experiment says to test milk left out 24, 12, 6, and 3 hours. Then every 6 hours for 2 days. Based on how fast the milk goes from blue back to white in color this shows how much bacteria is present. What would a control be? Please explain as much as you can. This is our first science project and we really are confused. And to make it worse she has just started at a new school and the class is already in the middle of their projects! Thank you SO much!