No, not an ill-considered Valentine's day gift*, although it sounds like it, but a little garment for a hot-water bottle. As I have said before, both the beauty and the utility of this mundane object are very much improved by knitting it a little woolly sweater in whatever style appeals to you. When newly filled it feels warm instead of hot, and it releases its heat slowly and is still warm in the morning.
A superwash wool or wool mix is excellent, but if for one reason or another you don't use wool, the main thing is to choose a fibre with good insulating properties. The point is to slow the release of heat; a lot of commercial hottie covers are made of fibres or weaves that don't deliver.
The photo is a bit rubbish because I took it in a hurry with my phone, sorry. This one was a present for my sister so I can't take another photo unless I visit her. It's made in Rowan RY Cashsoft Aran. It took quite a bit more than one ball. The dye lots vary markedly, but it feels great and not at all tickly, which is a bonus.
You make it as follows:
- Cast on as though for a giant's toe-up sock, 30 stitches across (for a fairly tight fit at my gauge), using Judy's magic cast on for toe-up socks, which you should learn if you haven't yet.
- Knit in the round (60 stitches to a round) without increasing until it is long enough to reach the shoulder of the bottle, introducing a moss-stitch heart or any other decorative element that pleases you and doesn't distort the shape. Mine has a heart on each side.
- Change to 1x1 ribbing and continue till it is long enough to conceal the top without covering it (I don't see any reason to cover the top - it doesn't lose any heat through there, and there's no need to take the cover off just to fill or empty the bottle).
- Cast off. To make the collar stand up straight as shown, use Techknitter's beautiful tubular cast-off for 1x1 ribbing. This will make the opening a lot less stretchy, but that's fine because an empty hot water bottle is flexible and can be partly folded along its long axis for insertion.
*For non-knitters: it is widely regarded as unlucky to knit any sort of sweater for any man-in-your-life to whom you are not closely related, or legally (or at least customarily) married. It is tempting fate, specificially tempting it to bring the relationship to a quick end.