The negatively charged comb induces
an opposite charge in the water stream and this opposite charge
in the water stream gets attracted to the comb. The mass has
to be small enought for the electrostatic charge in the comb
to move the mass. The static attraction force is relatively small
and thus larger masses are not moved. This is why the water stream
has to be small so it can be attracted by the charged comb. A
larger stream of water needs a much larger static charge to bend.
It is suggested not to get the comb wet.
The water would take away all the charges and it would be very
hard to get the comb recharged with static electricity. Other
materials that can be used instead of the comb are: a balloon,
hard rubber, or a plastic ruler. You might wish to experiment
further by trying other materials found around the house.