There are two possible reasons for what you
describe, depending on whether the balls seem
sticky in the water or outside the water. If
you're referring to the fact that wet things
tend to be kind of 'sticky' (dust or a hair will
stick to a wet surface, for example, and resist
being shaken off) this is the result of the
large surface tension between air and water.
Water interacts very strongly with itself, so
when exposed to most other objects
(hair/dust/etc), the water at the surface would
prefer to be in contact with the new object than
the air. This is why you can see water fighting
gravity by creeping up certain surfaces.
Alternatively, if you mean that the sponge
balls are stickier under water than they are dry
and in air, I suspect that this is likely a
feature of the sponge material itself. You can
do some experiments and see if they stick better
to different materials under water. Try glass
versus plastic, for example, and let us know
what you find!
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