White cat or black guinea pig?

My white cat and my guinea pig
are mutually exclusive:
Try and see them both at once -
one always proves elusive.

This is the only photo I took today - my camera happened to be within reach when I spotted the shadow of my elbow* looking very like a guinea pig. Didn't notice the cat till I looked at the image on-screen, and noticed that it works like that classic illusory image: Is it 'two talking heads in profile' or 'one central candlestick'?

*As far as I could tell!

Which did you see first?