Placing old sponges in the bottom of your potted plants is a great way to re purpose them.

The great thing about sponges is that they will absorb and hold excess water and slowly release it to the soil, essentially creating a home made self-watering pot. The sponge also holds dirt in (for those pots with larger drainage holes).  Here are some more clever reuses for household sponges. 
Source:  Good Environmental News Blog

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