Whether you see Albert Einstein or Marilyn Monroe (depending on your screen size and distance) tells you how good your seein' balls are.
I don't remember the first time I ever saw this illusion, but it was a long time ago. So long ago, I'm pretty sure it looks a little more Marilyn Monroe-y than the first time I saw it. That makes me feel old. On the other hand, when I go home for Easter this weekend, I will show it to my mom in the hopes that it will make her feel old, which will make me feel young all over again.
And if you don't know who Albert Einstein or Marilyn Monroe are, you're just too young. Period. Get off the Internet and do your homework.