Mr. Fox uses blueberries laced with tranquilizer powder to put the guard beagles to sleep. Each one is sliced in half, filled with 10mg of high-potency sleeping powder, and glued together with a brand of superglue called Wacko Stick. The powder comes from a plain, brown glass vial marked "Tranquilizing Dust". This is enough to tranquilize a gorilla. Mr. Fox uses a metal tube as a type of blowdart gun to fire the blueberries into Boggis's Chicken Farm.
Blueberry paw

"You forgot our blueberries? I did say to- I wrote it on your paw!"

The beagles eat the blue berries, and immediately fall asleep. When Mr. Boggis discovers one and eats it, he immediately falls asleep as well. During the rescue mission, Mr. Fox planned to bring more blueberries to soothe Spitz, but Kylie forgot to bring the blueberries.