<s>[INST] I was giving my 4 month old a bath and noticed a hard knot in the middle of his back near his spine. I have never noticed this before. It is not painful to touch for him. Also when I stand him up or sit him over it goes away, but I can still feel it when I run my fingers over the area. What could this be? [/INST] This is the tip of 6 th Cervical vertebra that you feel. The  6 th vertebra has the most prominent spine which is generally palpable. You will find in every child particularly in lean children. Its absolutely normal and nothing to worry about. Hope I could relieve your anxiety. </s>