To fill it, either pull out the plunger completely and pour the lubricant into the barrel straight from the bottle, or if your lube comes in a tub, dunk the entire assembly in the container and pull up the plunger to fill. Don't double-dip, and make sure to clean the barrel each time! The hypodermic-style construction guarantees that you won't spill a drop on your partner or on the sheets till you press down the plunger.
