Professor Apple picked it up and laughed. “This isn’t one of mine. In fact, these dark dots are drops of ink. And I would never leave purple smudges on a sterilized lab plate. Someone was playing a joke.”

Everyone looked at Cooper. He held up his hands. “It wasn’t me.”

Just then Pascal’s ears perked up. “Did you say purple smudges? Dr. Newton always carries purple chalk with him to use in his mathematical equations!”

“And look! The three clumps are the same size,” Jamal said.

“And they all have seven dots,” Chelsea added. “7, 7, 7.”

“That’s it,” Gabby said. “Not a joke but a clue. But to what?”

“Hey, I know,” Simon said. “It’s an address. The Rutgers University Golf Course is at 777 Hoes Lane. I think it’s across the street from here.”

“Why yes it is,” said Professor Apple. “I got some of my bacteria samples from the golf course pond.”

“We’re on the right track now,” shouted the Trackers as they ran for the stairs.

“No running in the halls,” called Mr. C. “And wait for us down by the bookmobile.”

“Thank you Professor Apple,” said Mr. C. “I hope we didn’t disturb your work.”

“Not at all,” smiled Professor Apple. “It was a pleasure to have future biologists in my lab. Good luck finding Dr. Newton.”