What an awesome and beautiful idea to do with the kids before Christmas! Sooooo nice!
You can use any citrus fruit you like; oranges, mandarins, limes, grapefruits or lemons. Then, you have two choices. Either cut the fruit carefully into thin slices or peel carefully the skin off the fruit.
If you choose to slice the fruits, arrange the slices on a lined baking tray and put the tray in a preheated oven at about 90-100°C. They should not touch each other. Bake them for about 2-3 hours, depending on how thick the slices are, while turning them on the other side every 30 minutes or so. You should check every slice separately and when one dries out, get it out of the oven and let the others dry some more. Let them cool down completely and then pierce a hole in them. Now you can use anything you like, some yarn, cord or a nice ribbon, depending on the use you are planning for them. Even paper clips or ornament hooks could do the trick.
If you choose to use the peel, take some nice cookie cutters, like for example a star-, moon-, tree- or heart cookie cutter and cut the shapes out. Now, do the same as above. Isn’t it easy?
You can use also some spices or essential oils for more aroma, and even decoration!
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