Rajnikanth had spit on it

 Once a boy took his girlfriend to the Red Fort.
.
Girl-Why is the Red Ford red?
.
Boy was in a pretty jovial mood.
. Boy-It is because once Rajnikanth had spit on it
after eating 'paan'.
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The boy now takes her to Taj Mahal.
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Boy-Now don't ask me why Taj Mahal is white.