Some of the contents of a bottle can get in between the stopper and the inside of the bottle neck. The liquid dries and sticks the stopper to the bottle. Foreign matter such as dirt and dust can also make the stopper stick inside the bottle. Or, the stopper can get stuck because it was forcefully pushed into the bottle neck. Whatever the case, with a little time and effort you can get the stopper unstuck.
Lay a towel in the bottom of a sink to help protect the bottle from breakage if you accidentally drop it. Turn on the faucet so the water is at a warm temperature. Hold the bottle neck under the stream and turn it slowly.
Move the bottle over so the warm water stream is directed on the stuck bottle stopper. Turn the bottle so the stopper gets completely warmed.
Raise the water temperature up and continue to heat the bottle neck and stopper. Keep raising the temperature up a little at a time and warming the glass until it's as hot as you can stand it on your hands.
Hold the bottle firmly with one hand while you carefully attempt to wiggle the bottle stopper with the other hand. If the stopper moves, try to pull it out of the bottle neck.
Remove a stubborn stopper by cleaning the part where the stopper and the bottle meet with the bristles of a toothbrush. Insert the thin tube provided with a can of WD-40 into its nozzle. Lightly spray the part of the bottle where the stopper and bottle meet with the lubricant.
Heat the bottle neck and the stopper again with hot water. Then, clean the area with the toothbrush to remove loosened dirt and other particles. Give the place where the bottle and the stopper meet another application of WD-40.
Wiggle the stuck stopper again and remove it from the bottle. You may need to reheat the glass, clean the neck area with the toothbrush and applying more WD-40 to it before it finally releases.
Don't try to unstick a stopper if the bottle is cold. Allow it to warm-up to room temperature first to avoid cracking or breaking it.