The phrases below have something in common. Can you determine what it is?

NOBLE GAS 

JUNGLE GYM

MIDDLE GROUND

MISSILE GUIDANCE

PERISHABLE GOODS

 

HINT:

The puzzle title is KNEE DEEP.

 

Answer:

 

The word LEG, in the middle of each phrase, serves as a "joint" between the two words, as the knee serves as a joint in the middle of the leg.

 

nobLE Gas

jungLE Gym

middLE Ground

missLE Guidance

perishabLE Goods

 

Credit:
Harvey Estes 

KNEE DEEP