From the Super Mario Wiki
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Metal balls, also referred to as Pinballs,[page number needed] are stage obstacles in Waluigi Pinball from Mario Kart DS, Mario Kart 7 and Mario Kart Tour. They roll around all throughout the course, and flip over any player that drives into them. As such, they act similar to boulders and Big Steelies. On the pinball table section of the track, metal balls are bounced around by bumpers and flippers.
Pinballs also appear in Mario Sports Mix on the Waluigi Pinball court, which based on the Mario Kart track of the same name. Here, they are released onto the court by a giant robotic Waluigi and then continuously roll around the area, hitting bumpers and walls. When they collide with a player, they push them back and stun them for a few moments while also becoming faster. They keep their momentum if they hit a bumper but will revert to their original speed if they hit a solid wall instead. If a pinball ends up in the hole at the top middle of the court, another one will appear shortly afterwards.