The Buchner funnel is used in vacuum or suction filtration in order to separate solids from liquids. This is used with a piece of filter paper covering the holes. Wet the filter paper with the solvent you are using to create a seal before filling it with the solution to be filtered. Sometimes you must press the funnel down onto the top of the vacuum flask to create a good seal. Other times, when you have a mushy, damp product, it can help to leave the funnel in place and continue to pull air through it until the product dries slightly.