A fuel additive is directed to preventing slagging, a phenomenon caused by the ash in a fuel particularly during the combustion of a fuel having such a large ash content as is typically found in coal or oil coke, for example. This method allows a user to provide supplementary data during an interactive teaching procedure. A first key is defined by the first switch and the first contact region and is actuated by deflecting the first contact region to actuate the first switch. The grip is inserted into the slits, and both end portions of the grip are fixed.