The four spheres (which the artist calls baubles) appear to be moving.  If you move your eyes rapidly around the image, you will find that the motion effect is enhanced.

A larger, single bauble version of this image can be found below.

To see more motion illusions from Kaia Nao, please check out Floating Star Illusion and Schmear.

(via Kaia Nao)

Other Optical Illusions You Might Enjoy

Frazzle by Kaia Nao

18

Schmear by Kaia Nao

18

Ambiguity by Kaia Nao

18

Caterpillar Race by Kaia Nao

18

Floating Star Illusion by Kaia Nao

18

Tunnel Vision by Kaia Nao

18