|Image source here.|
The Babinski test consists of someone stroking the lateral side---of the bottom of the foot, going in the direction of the heel to the toes---with a blunt but somewhat sharp object. (If you want to try this test at home, perhaps you could use a capped pen or a smooth chopstick? Doctors often use the end of their reflex hammer to preform this test).
When the foot is stroked in this matter, the normal adult reflex is for the toes and foot to all curl downwards (a motion termed plantarflexion), as seen in image A.
However, in people with upper motor neuron lesions of the corticospinal tract (i.e. people who have had a part of their spinal cord damaged), there response will be much different. Instead of the toes curling, they will fan outwards, and the big toe with extend backwards (a motion termed dorsiflexion), as seen in image B. If patients are experiencing neurological or motor abnormalities, the Babinski test can be preformed to potentially narrow down the location of the spinal cord problem.
The coolest part of the Babinski test is that when it is preformed on babies, their toes will fan out just like image B. This is because their corticospinal pathways haven't fully developed yet (they are not yet myleinated). In the healthy baby, this abnormal reflex will disappear sometime before his or her second birthday.
Any readers with babies should give the Babinski test a try. It's a reflex that your babe will only have for so long!