When two spring balances joined at their free ends, are pulled apart, both show the same reading. Explain.

The spring of balance A pulls the spring of balance B due to which we get some reading in balance B. The same reading is seen in balance A because the spring of balance B also pulls the spring of balance A by the same force.

The pull on the spring B by the spring A is the action F_BA and the pull on the spring A by the spring B is the reaction F_AB.

This demonstrates that "to every action, there is an equal and opposite reaction" (i.e., in magnitude F_AB = F_BA but they are in opposite directions. )