Shorts + Pants = Shants! Better known as convertible pants or trekking pants, these types of long pants zip off around the knee and turn into shorts. A new pair cost anywhere from $40 - $80. Mine cost under $10 and I made them for my 11-year-old son, Liam.
I bought these boys size 10 Old Navy pants at my local thrift store for $4. The zippers were on sale for under a couple of bucks each and I had some thread that was close enough in color.
I found this fantastic video, DIY convertible zip-off pants, that shows how this process works.
Here's what mine looked like before I hacked them up:
Liam did not like the pockets on the side, so I took those off.
After I removed the outside pockets, I measured down about 14" from the inseam, drew a chalk line and cut the legs off.
Next, I turned the raw edges under and stitched to create a finished edge. If you're a ninja stitcher, you could do this while adding the zipper, but I like to do this in 2 steps.