From the Super Mario Wiki
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Spike Ball is an ability learned by Bowser in the game Mario & Luigi: Bowser's Inside Story. The ability is learned when Mario and Luigi cure Bowser of his back pain by drilling into the center of the Lumbar Nook. When they finish the process, Bowser's back loosens; he becomes more flexible and can use the Spike Ball ability. This ability allows Bowser to roll up into his shell, become a ball and climb walls of soft dirt, smash through certain objects, get shot through a spring and activate treadmills in the overworld.