Plastic bags can be welded together with a soldering iron. The resulting seam is strong and water resistant. This is a very useful technique for making kites.

Step 1: Tools

You need some oven paper and a soldering iron.
I use a soldering iron with a flat tip and temperature control.

Step 2: Stack Materials

Place the first piece of plastic on some oven paper and mark the place where you want to make the seam. Then place the other piece over the first one aligning it with the mark.
The plastic sheets are slippery and they like to escape. You can make them stick in place by wetting them just a bit.
Now place another piece of oven paper on top of the plastic. You should be able to see the markings through the paper. This is why it is a good idea to use light colors and avoid black plastic.

Step 3: Welding the Seam

With very little pressure on the soldering iron move it along the seam to melt the plastic sheets. It is important that you hold the tip so that the whole surface of the tip makes contact with the paper.
The right speed depends on the thickness of the plastic. Low density plastic is easier to weld than high density because HD is thinner.

Step 4: Done!

When you have finished the seam, inspect it and test it by tearing it apart.
After some practising you will know the right speed and presure for different materials.

Use your new skill for making kites or inflatable stuff or something!