I often pour one glass of cider and recork the larger bottle for later. This has one drawback: no longer do I know what type of cider inside the bottle. Without caps, my blank bottles are impossible to tell apart.
My solution? With a magnet and a steel ring, I can attach the cap to the neck of the bottle. These things are require no tools to make, and they are infinitely reusable. I keep several stuck to the fridge in case I have a bottle-labeling emergency of some kind.
How to make one: Stick the magnet on the ring, and drop the ring around the neck of the bottle. The magnet will hold the cap there as long as you like. Magnetic or ring-based charms could also help people identify their bottles at a party.
Where to get the parts: I used magnets from hard drives because they’re powerful and semi-elegant, but any magnet that keeps the cap from flying away under normal drinking conditions should be fine. As for the rings, large ones work best — they need an internal diameter of at least 1.1″ (28 cm) to fit past the flared mouths of most bottles. Split rings (a.k.a. key rings) are fine if they’re large enough. Hardware stores also have a number of cheap plumbing fittings that would work as long as they’re ferromagnetic (made of metals like iron or steel) and will attract to a magnet.