Q: Why are Christmas trees like bad knitters?
A: They both drop their needles!

Explanation: Christmas trees that are brought into the house often lose their needles; they fall off the branches as the tree dries (even if it gets enough water).  When someone knits a scarf or a sweater they use knitting needles, the long sticks that knitters use as they knit one and purl two.  This joke is funny because it plays on the double meaning of the word “needles.”