Pop out a shape out of another shape
For simple shape and best result you can do it frame by frame, drawing each morph state separately
For more complex shapes, I will advise:


Move second object (and maybe scale it from little size to normal) from behind first one
Bend part of first object outwards slowly, while new object appear (simultaneously with point 1)
In the moment, when second object is fully separated from first, first object shall rubber bend back with bash, as cartoon character, who was hit with the bat in the head, or as water surface, after water drop hit it.



To fully understand, what I mean in last point, look at this: