I enjoy buying single skeins of yarn on clearance and seeing what I can do with them. One example is this silk-like acrylic I found for $1.25.
When I have a small amount of yarn, I do scarves or shawls in a triangle shape. With this yarn, I begin a half granny square at what will be the center of the longest side (the base, in geometry terminology) of my triangle. The legs of my isosceles triangle will grow by by increasing at the beginning and end of each pass.
The crocheting will continue until I no longer have enough yarn left for a complete pass.
I am also continuing a knit triangle with a single skein of lace weight alpaca. This triangle began at one point with a few stitches. It grows by yarnovers at each end on every other row, creating a diagonal of tiny holes along the borders.
When this shawl is done, I will wet it and stretch it to open up the lace.
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