Plastic cups made from used plastic bottles.
Step 1: First Step: Cut the Bottles
Cut the bottles separing the top and bottom of those. The top will be the part that will contain the your liquids, so make it bigger or smaller as desired. the bottom part will be the base of the cup, so you would have to cut it without differents heights to have a stable base.
Step 2: Step 2: Assembly
Cut the center of the bottom of the bottle and place the top of it inverted through the hole and then close it.
As seen on the pictures, you can cut it with scisors or with a soldier to melt the plastic if it is too strong to bbe cutted with scisors.
Step 3: Final Step:
You can now fill your cups and drink from them.
If the plastic is too sharpen (or it has been problems on cutting it) and is harmful to use it, you can just melt a little bit the edges where your lips are placed so them get rounded.