What is a lumbar puncture like and how come you have to lie on your back afterwards?

Spinal tap: This is a procedure for evaluation of the fluid that bathes the spinal cord and allows the brain to float. It can be done with or without x-ray. You have to lie on your back because the hole that is made by the needle to draw the fluid out needs to seal. If you sit up, gravity will allow too much fluid to drain and your brain will sink. This may cause bad headaches and occasionally brain bleed.