Yes, you can take unopened tennis balls on a plane, but there is a specific physical phenomenon you should be aware of regarding pressurized cans. Tennis balls are sold in pressurized containers to maintain their internal bounce; when you fly, the aircraft cabin is pressurized to an altitude of approximately 6,000 to 8,000 feet, which is a lower pressure than at sea level. This pressure differential can cause the pop-top seal of a new can to bulge or, in rare instances, hiss or pop during the flight. While the TSA and international aviation authorities do not prohibit them in either carry-on or checked luggage, many frequent-traveling tennis players prefer to pack them in checked bags to avoid any minor startled reactions from security or fellow passengers if a seal fails. If you are a high-performance player, note that the pressure change might slightly affect the longevity of the ball's internal pressure once the can is eventually opened at your destination, though for casual play, the difference is negligible.