Does anyone know how splat toys work?

I saw someone demonstrating them in the local shopping centre and I was fascinated. You throw the thing on the ground and it forms a puddle. Then it reconstitutes itself spontaneously as if by magic.

I tried DDG and it seems they are actually thin plastic bags filled with soapy water but I couldn't find an explanation of how the shape is recovered. My guess is something to do with surface tension.