The World's Best...

So, the other day I mentioned that I got a new t-shirt that says I'm the world's best. But today I went to see my friends, and we went outside to play in the garden. It was nice and sunny, so I got to wear my t-shirt again. And when I turned around, my friend Elliot told me that my t-shirt had something else written on the back.

Apparently I'm not just the world's best bunny...

Can anyone explain to me what that means?