What is a couple? Give its illustrations.

(N/A) couple is a pair of equal and opposite forces acting on a body with different lines of action. $A$ couple produces rotation without translation.
Illustration $1$:
As shown in figure $(a)$,when we open the lid of a bottle by turning it,our fingers apply a couple to the lid. The lid is in translational equilibrium (net force is zero) but not in rotational equilibrium.
Illustration $2$:
As shown in figure $(b)$,the Earth's magnetic field exerts equal and opposite forces on the poles of a compass needle. The force on the North pole is towards the north,and the force on the South pole is towards the south.
When the needle is not aligned in the North-South direction,the lines of action of these two forces are different. Hence,a couple acts on the needle due to the Earth's magnetic field,causing it to undergo rotational motion without translational motion.