Water Bottles?

recycle them maybe?

or you could use them for something else...for example, my grandmother often buys those microwavable soups in the plastic bowls, and instead of throwing them away she grows radishes or peas or other vegetables or flowers in them during the winter so they'll be ready for spring. you could do the same thing by cutting the water bottles in half

or you could make a bunch of CO2 apparatuses out of them
 
you could build co2 generators out of some of them and recycling the rest.

Or you could make DIY filters out of them.

All you need is the bottles, media, and some way to draw or push water through them (such as a power head or a sponge filter type air stone set up).
 
Or you could reuse them.
 
melt them down and pour them through a funnel and see if you can create your own epiweb.
 
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