Question: > 
> There is a group of finite aliens on a spaceship. Show that there are at least $2$ aliens who know the same number of aliens on the spaceship. 
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I was given a hint, and that was to use the pigeonhole principle. I think I can visually see it but I am unsure how to show it.

Answer: 
HINT: Let $n$ be the number of aliens on the ship. As noted in the comments, you must assume that $n\ge 2$. Suppose that A is an alien on the ship; how many of the others can it know? The largest possible answer is $n-1$, and the smallest is $0$. 

* How many possibilities is that?
* Is it actually possible for one of the aliens to know $n-1$ of the others and another to know $0$?
**Hint:**

There are $N$ aliens, and assume one of the alien knows $K$ aliens. $(N>K)$

Can each alien know **distinct** number of aliens? 

Assume $A\_1$ knows ${A\_2,A\_3 \dots A\_{K+1}}$, now that $A\_2$ knows $K$ aliens, the aliens who he knows also knows him.(Though confusing, its the fact -evil laugh-)