Your spine looks something like this, from the side. The front of the spine is the smoother side on the left, the back of the spine is on the right where the dinosaur-like protuberances, called spinous processes are located.
If you run your fingers along your spine (or your friend's spine) in the back you will feel these bony landmarks running from the base of the skull to the top of the pelvis.
The cervical spine is the neck, thoracic spine is the ribcage area, and lumbar spine is your low back which meets your pelvis. The sacrum is at the back of your pelvis, and the coccyx is your "tailbone".
Start thinking about how yours might look or feel in your body as you move and breathe. Can you visualize your own spine?
Perhaps you can sense how to lengthen you spine more clearly if you can "see" it in your own body. Try this next time you practice & perceive how this might change your experience of your own body and mind as you practice.