When you fly somewhere, the two things you have to make sure you put in your carry-on are any medications you need and a change of clothes in case your luggage gets lost. Here's one more thing you might want to toss in there.

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A writer for Travel & Leisure did an article on why she always travels with a tennis ball in her bag. Here's why and anyone with a bad back might already know the answer.

A tennis ball is really good for giving yourself a massage when you're sitting down. Which is important, because all that sitting wreaks havoc on your back, and your leg muscles.

Even if you don't have a bad back, it's a good idea because it increases blood flow, which can help with jet lag. Plus it's small, so it's easy to fit in your bag.

Just put it behind your back, lean on it, and roll it back and forth. You can massage yourself by rolling it across your shoulders, legs, and feet. You should also still get up and stretch if it's a long flight.

If you don't bring a tennis ball, you can still give yourself a quick leg massage. A lot of people's legs swell up when they fly. So start at your ankles and work your way up to your knees.