So, what do you need? You need to open your fridge and clear all the jars that have been standing there for months, that's what you need... But don't throw them- here's how you can use them again, and I can promise you, you will enjoy the result.
Wash them thoroughly and remove the labels. Now go to your basement, etc. and find out the leftover wall- or furniture paint that you have. (I used some lightblue latex paint that we've had for 5 years at least- it's still good).
Before you put the paint in the jar, you might want to make it a bit more interesting, and add a label on the inside. I used a scrapbooking clipart/sheet with some vintage labels, but they are just as easy to find on the web (search for "vintage labels" on Flickr, for example) and print.
Make sure you use good glue (in my case- glue with medium, suitable for glass).
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When you are satisfied that the label is glued, proceed with the paint. Pour a bit of paint at the bottom of the jar (about 1/7th of the jar volume) and tip it to one side. Start roling the jar, tipping it even more, to coat evenly the surface of the jar.
Keep rolling and tipping until you reach the rim, then turn the jar upside down and leave it for some time on a tissue paper or a piece of paper.
When the paint stops dripping and before it is completely dry, you might want to clean the "neck" of the jar and leave it transparent (I did it in this case), or you might leave it as it is.
When the paint is dry, your new vase/flowerpot is ready to use.
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And the final result:
I think it's cute. :-)