Why we use "-ing" form (cleaning) after the word "to"?

[Zishuli]

English Language Tests, Intermediate level

ESL/EFL Test #159 "Digital cameras, photography and picture vocabulary", question 4

One key to cleaning the ......... of your camera is not to touch it with your fingers.

(a) zoom
(b) eyelet
(c) lens
(d) tripod

English Language Tests, Intermediate level

ESL/EFL Test #159 "Digital cameras, photography and picture vocabulary", answer 4

One key to cleaning the lens of your camera is not to touch it with your fingers.

Correct answer: (c) lens

Your answer was: correct
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Hi, I am not quite sure why we use "-ing" form ( cleaning) after the word "to" in this sentence. Can anyone make it clear for me?

[Alan]

Hi,

This use of 'ing' after 'to' is really an indication that what you are really saying is: This is the one solution we need for cleaning ....

Alan
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English as a Second Language
You can read my ESL story Briefly

[Nghung]

Hi. If we don't touch the lens how can we mount it to the body?!

[Mister Micawber]

.
You touch only its housing.
.
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Native English teacher at Mister Micawber's