For a lumbar puncture, you lie on your side with your knees drawn up toward your chest. This position helps widen the spaces between the bones of the lower spine so that the needle can be inserted more easily. A numbing medicine (local anesthetic) is put in the skin. Then a long, thin needle is put in the spinal canal to collect a sample of cerebrospinal fluid (CSF). Your doctor may need to move to another area of your spine if it is hard to get to the spinal fluid.