Doubt in pigeonhole question

I want to know that how the probability is 1.
please explain by taking some cases
so that i am able to understand easily

General pigeon hole principle is that if you have 6 pigeons and 5 boxes, then atleast one of the box will contain more than one pigeon!, the probability of such event is 1.

sir please ek baar example de ke samjha dijiye. kyuki sir aisa bhi to ho skta h ki sir 2 pairs mil jaye jo ki 5 s divisible ho kyuki ek s jyada pair bhi exist kar sakte h

sir mujhe refrence k lie kuch link provide kar dijiye


is helpful
also see
https://www.whitman.edu/mathematics/cgt_online/book/section01.06.html

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