Skull switch

A Skull Switch.

Skull Switches are items found in New Super Mario Bros. In total, the switch is found in three boss fights; the first with Bowser, the eighth with Dry Bowser, and the final battle with Bowser and Bowser Jr. In the aforementioned boss battles, Mario's goal is to avoid the attacks of the bosses and hit the Skull Switch, causing the bridge which Bowser is standing on to collapse and send him plummeting downwards to whatever may lie beneath. When Mario sets foot on the safe platform with the switch, Bowser/Dry Bowser will run towards him and try to toss him back onto the bridge.

The Skull Switch acts as a replacement for the Ax, from the original Super Mario Bros. and in New Super Mario Bros. Wii, the Skull Switch is replaced by ! Switches.