Optical illusion?

I took this for a festive reason. A not-necessarily-unrelated fact: today is our younger daughter's birthday. The last one that begins with a "2."

But the reason I'm posting this is the title: there is perhaps a bit of an optical illusion here. The camera/photo was level. The level of the beer in the glass is horizontal. The letterings on the bottle labels in the background are level. So what makes this look "off"? I think it's the base of the glass. But as long as the beer is tasty, who cares, right?

Happy Birthday, Grace!

1 year ago (“Ellen & Larry and Clare”): http://365project.org/rhoing/365/2014-12-30
2 years ago (“Time waits for…”): http://365project.org/rhoing/365/2013-12-30
3 years ago (“Proofreeder Wanted!”): http://365project.org/rhoing/365/2012-12-30
4 years ago (“Another family’s toy train tradition”): http://365project.org/rhoing/365/2011-12-30
5 years ago (“Grace’s birthday”): http://365project.org/rhoing/365/2010-12-30

[ IMG_8099S12x8tm :: f/5.6 :: 1/10" :: ISO-1600 :: 55mm ]

30th December 2015 30th Dec 15