This one's animated, so go and check out the interactive Pigeon Neck illusion at Michael Bach's website.
In the animation above you see "pigeons" craning their neck while scurrying along, and worms shrinking and extending while worming their way. Looks a little silly…
This was Jun Ono's, Akiyasu Tomoeda's, and Kokichi Sugihara's contribution to the 2014 "Best illusion of the year" contest. As these authors write, it is a version of the "kickback illusion", which is closely related to the Stepping Feet illusion – see my explanation there.
Of course, they aren't really craning their necks. Not in this consensus reality, anyhoo.