Metal ball

Metal balls, also referred to as pinballs, are obstacles from the Mario Kart series. They have a similar design to iron balls and also act like them.

Mario Kart series
Metal balls are obstacles in Waluigi Pinball in Mario Kart DS, Mario Kart 7, Mario Kart Tour, and Mario Kart 8 Deluxe. In Tour, they also appear in GBA Bowser's Castle 4. They roll around all throughout the course, and they flip over any player that drives into them. As such, they act similarly to boulders and iron balls. On the pinball table section of the track, metal balls are bounced around by bumpers and flippers. In GBA Bowser’s Castle 4, they are spawned by a propeller-like contraption, and bounce around in a straight path. In Mario Kart DS, they are 2D sprites that always face the camera, while in Mario Kart 7, Mario Kart Tour, and Mario Kart 8 Deluxe, they are 3D models. In Mario Kart Tour, they show a reflection of the pinball table section of the track, regardless of where the metal balls are. They are black in Mario Kart DS and Mario Kart 7; however, they are gray in Mario Kart Tour and Mario Kart 8 Deluxe.

Mario Sports Mix
Pinballs also appear in Mario Sports Mix on the Waluigi Pinball court, which is based on the Mario Kart course 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 they 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 appears shortly afterwards.