HAPPY BUBBLE FACES
by Anton J
This tessellation shows translation (a.k.a. slide, glide) symmetry.
The square-ish "tile" (tile = repeating shape) with wavy sides is very neatly, precisely reproduced and fits with all the other square-ish tiles perfectly, without gaps or overlaps. That's what a proper tessellation does.
However, there's air in there. Air can be any shape. It's filler... just filling the gaps around the faces. There's no challenge in making it fit. That's why this tile shape creates a valid abstract tessellation, but when we put in faces that don't completely fill the spaces, it is not an Escher-style tessellation.