Even the most dedicated wine lover can end up with a half bottle of wine at the end of the night. So should you toss it or keep it? If you canmake a small effort, just keep it. But if it’s going to sit on a warm kitchencounter for a few days, toss it. Here’s a little science before I get to the“how-to’s.” Wine “goes bad” once you pull the cork because of oxygen. Thesecond oxygen hits the wine, it starts to, well, oxidize. You can slow down theprocess but you’ll never completely reverse it. That said, you can keep abottle for a few days without it turning to vinegar. After a week, it’s time totoss it. (And I mean toss it; contrary to popular belief, cooking with spoiledwine will not add flavor to your dish.)

Wine gas: Inert gas willkeep your wine fresh for days. (Inert gas is essentially pure air that containsno oxygen, and it prevents oxidation of the wine.) The two most common brands,Wine Life and Private Reserve, come in cans that feel empty. But really the gasinside is lighter than air and replaces any oxygen left in the bottle with thegas, preserving the wine. Just spray and re-cork and the wine will stay freshfor at least a few days and up to a week (and some say up to a month). You canfind both brands in wine shops and online for around $10 a bottle; one bottleshould be good for about one hundred uses. If you want be fancy about it, thereis a “wine preservation steward” that replaces the oxygen with argon gas thatsells for around $200 and keeps wine fresh for up to three weeks.

Wine vacuum: A vacuum pumpdraws out any oxygen in the bottle and seals it, preserving the wine for 2 to 3weeks. You just place a stopper over the top and pump out the oxygen. They costanywhere from $10 to $50. (But a vacuum won’t work on sparkling wines.)

Half bottles: An easy andcheap way to preserve your wine is to pour it in half bottles (375mL) andrecork or use a wine stopper. The size of the bottle eliminates excess oxygen.Our winemaker friend swears by it. (For even fresher wine store the half bottlein the ‘fridge; see below.)

Therefrigerator: Several winemakers I know stick recorked, half-emptybottles of wine in the fridge, both whites and reds. It slows down the deteriorationof the wine. Just pull reds out of the fridge about an hour before you want tofinish the bottle.

Bubbly: Obviously Champagne and sparkling wine pose additional problemsgiven the CO2 involved (it’s impossible to preserve all those bubbles), but aninexpensive Champagne stopper will keep itovernight. Any longer than that just cook with it; leftover Champagne makes a great white sauce forchicken, fish, or pork.

Screwcap: If the bottle has a screwcap instead of a cork,you're one step ahead. A good screwcapactually preserves the wine better than a cork ever could. So once you unscrewthat top, just screw it back on tightly (and even better, stick it in the‘fridge) and enjoy the wine for up to a week.