paper chromatography experiment: why should we tightly cover the beaker with aluminum foil?

light bond plus aluminum Transformers For Lights In Ceiling Aluminum Light Post Reynolds Aluminum Foil Recipes Proper Way To Use Aluminum Foil Heating Aluminum Foil Shiny Or Dull Side Of Aluminum Foil Smoking Dull Or Shiny Side Of Aluminum Foil Baking Aluminum Foil Aluminum Foil Food Safety Insulated Aluminum Panels Drywall Aluminum Studs Eating Aluminum Foil Reynolds Wrap Aluminum Foil Reynolds Aluminum Foil Cold Forming Aluminum Light Bond Plus Aluminum Gold Aluminum Foil Aluminum Foil Shiny Side Aluminum Foil Bowl