When pillows are placed in the dryer on their own, the materials inside can clump up and cause your pillow to become flat. An old-fashioned method for preventing this is using multiple tennis balls in the load. As the dryer spins, the tennis balls hit and fluff the pillow while it dries. This method is not necessary, however, to clean and dry your pillows.
A tennis ball can extend the life of your pillow. When the pillow dries normally in the dryer, the cotton or feathers clump together. With the use of one or more tennis balls, the free-roaming ball bounces around and hits off the pillows. Generally, you should use two tennis balls for every pillow in the dryer to get the most fluffiness.
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If you use your dryer on high heat, you're better off not including tennis balls with the load. High heat could cause the tennis ball to become overheated, leading to a rubber smell in the dryer. Also, the yellow dye on the tennis ball could transfer onto other objects if it gets wet and heated enough.
Instead of using a tennis ball, other objects can produce the same results. Tie a couple of T-shirts into balls and put them in the dryer with a single pillow. Add in a single clean shoe with multiple pillows. Small stuffed animals without any plastic parts can fluff the pillows and keep the dryer quiet.
Air-drying pillows avoids dryer use and tennis balls altogether. Remove the pillows from the washer and shake them out vigorously. Lay them out on a clean surface in the sun. The natural air drying allows the materials go back to their original fluffiness. After a couple of hours, shake the pillows again and replace the pillowcases to complete the process.