Blue Key (Mario & Luigi: Bowser's Inside Story)
From the Super Mario Wiki
The Blue Key is a key found in Mario & Luigi: Bowser's Inside Story and its remake. It can be found in the second floor of Peach's Castle. It is carried by a Fawfulcopter which Bowser chases after. In the southernmost room on this floor, Bowser must push himself as a spike ball against a large spring to shoot himself to the Fawfulcopter. When successful, he knocks the Fawfulcopter and gets the key which allows him to proceed further.
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