Well, actually you can. Just don't expect the wine to last too long thereafter. However there are many people that feel just by re-corking an opened bottle and keeping it cold it will be just fine for days in the fridge... not so much. As soon as the bottle is opened air will enter into the bottle. The longer the bottle is exposed to that air, oxidation takes place. So if you are planning on just keeping the wine for maybe a day after opening it, there is a chance it may be alright if you just pop the cork back in and stick it in the fridge. But it is always safer to either use a vacuum pump and stopper, or even give the bottle a spray with the Private Preserve before putting the cork back in and placing it in your fridge for safe keeping.