Nubbin

A Nubbin is a pimple like protrusion that is found throughout Bowser's body. In order for it to activate, Mario must hammer this object in order to make something good happen, like opening blocked areas, fire cannons, make platforms appear, etc. They seem to be mostly red to match the tone of the flesh inside of Bowser, but sometimes a different color scheme (for example: purple) will appear, which can cause a different effect from the regular Nubbins (like powering up Bowser). There is also a Nubbin in the Funny Bone; however, this one is large and yellow, and it will awaken Bowser from his knocked-out state.