Plant Gels + or - soil?

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So my mom and dad had bought me these little balls that are colored and made of this gel thing that absorbs water. You put themin your palnt so that you dotn have to water it as often. Does anyone have these or has anyone seen these before? They are quite cool and they work well as far as I cn see (as well for just decoration). But Im not sure if you put them on top of the soil fior plants or instead of soil. They dont seem like the kind of things that would work with soil.