This is pretty open-ended. How about:
[tex]\sqrt 2[/tex] and [tex]\sqrt 3 [/tex] are similar because they're both irrational, meaning neither is the ratio of two integers.
[tex]\sqrt 2[/tex] and [tex]\sqrt 3 [/tex] are different because [tex]\sqrt{2} < 1.5}[/tex] and [tex]\sqrt{3} > 1.5[/tex].
