The bags were fused together to create a thicker, more durable plastic material. However, it wasn't until recently that I discovered how to do this myself with my own shopping bags. It really is quite simple.
In addition to an iron, all you need is waxed paper, scissors and a bunch of plastic baggies.
Cutting off the bottom of the bag and the handles allows the bag to be opened up more, creating a larger finished piece.
You'll want to turn the plastic inside out so that the ink from any printing on the bags stays contained and doesn't get all over your iron. Using two bags folded in half (about 8 layers of plastic baggie) gives you a nice thickness.
Sandwich the stack of baggies between sheets of waxed paper and iron each side (on a medium/high setting) for about 20-30 seconds each. You may have to play around with the iron setting and the length of ironing time.
If you can feel the sheet "crinkle" or can feel the baggie layers sliding over each other, simply iron it again. The final sheet should feel like one thick layer.
Peel back the waxed paper to reveal your fused plastic sheet!
Hmm... I wonder what kinds of patterns I can make with different bags from different stores...