Just posted my last post and I noticed that I’d typed it’s when I meant its. This is a common error that I pick up my students on all the time, but it’s one that I make myself quite frequently.
[Its](http://en.wiktionary.org/wiki/its) is the possessive adjective form of the pronoun *it* used in the post in question as “YouTube has changed **its** user interface”. [It’s](http://en.wiktionary.org/wiki/it’s) is a contraction of *it is* as used in “*First impressions*: **it’s** [that is YouTube’s new interface *is*] certainly much less cluttered”.
Student beware: this type of error won’t be picked up with a spell checker … it might be picked up by a grammar checker … but don’t count on it. Since *its* is actually quite rare, as things don’t usually possess other things, it’s probably *it’s* that you want. But if you are unsure, ask yourself “is it its or is or is it it is?”