"to" is not the "to" that forms an infinitive there, but a simple preposition. Think of "secret to" as noun and preposition which go together as a unit. This unit is followed by a noun or the gerund form (-ing form) of a verb, which is the noun-like form of the verb.
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CalifJimRemember: "to" is not always the particle that creates an infinitive, and it is not always a preposition; sometimes it is one; sometimes it is the other.The trouble is: How can I know either it is a preposition or it creates the infinitive?
MikaelI can't differentiate it!Until you begin to get a feel for it, you'll have to remember the expressions that do not take the infinitive.