[Logic] Find the enssence of the question

A very simple question, but need to find a good start point :

Question : can you be sure that you can find tow dogs have the same number of hair in America ?

Ok, I got really confused in the beginning, but if I asked you the second question, you will got clear :

Question 2 : can be sure that in 400 people, you can find two people with the same birthday ?

Yes we can, because all birthday is from 1 to 365, but we have 400 people, so it's obvious.

Then come  back to the first question :
can you ? let's try to think  with the same thought :

how many possibilities for the numbers of hair of a dog : one 1cm2 we estimate 100 hair, the skins of a dog can be changed from 1 m2 to 2m2, so we calculate with the bigger one : we have 2 000 000 possiblities

then, how many dogs we have in America : we estimate every foyer has 1 dog, how many foy? : population of America devise by 4 : 318 000 000 / 4   ~  80 000 000.

  so now it's clear : 2 000 000 possibilities in 80 000 000 numbers of dogs :
we are sure that we can find two dogs with the same numbers of hair !



Try to think the essence of the question, but not just try to guess.
the essence is that the number of possibilities and the number of simples.