When you first buy a sponge it is very soft and then after a few uses it gets hard why?

Answer 1

two reasons:
1)Many sponges have a type of softer applied at the factory as you uses sponge you rinse the softener out.
2)Tap water contains various minerals, as the sponge dries the minerals harden.

Answer 2

you wash away the chemicals that keep the sponge soft. Also bacteria begins growing on the sponge which causes it to harden as well as smell.

Answer 3

You need to lay it on a dish or stand that has little holes in it so that the sponge can dry out by air going through it. Even a wire rack will do.