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Choose a suitable yogurt drink or probiotic drink container. The sizes of these little containers vary somewhat, so select the drink bottle by testing whether the flashlight will sit on the mouth of the bottle without falling into it. Also choose a suitable flashlight; it should have a strong beam but should not be too expensive, as the duct tape will likely ruin its finish when you decide to pull the contraption apart later (which will be essential if traveling, as you don't want the contents of the bottle sloshing onto the flashlight).