A cat lifts up her kitten by its neck.
A bird carries a worm to its nest.
A horse pulls a wagon along a road.
Step-by-step explanation:
Work is defined as the product between the force applied to an object and the distance the object has been moved:
therefore, work is non-zero only if the object has been moved by a distance different from zero. This is exactly what happens in the three following scenarios:
A cat lifts up her kitten by its neck.
A bird carries a worm to its nest.
A horse pulls a wagon along a road.
In all these cases, an object (the kitten, the nest and the wagon) are moved through a certain distance, so work is done. On the contrary, in the other two examples:
A dog rubs his back against a large tree
A goat butts its head against the barn wall.
No object is moved, so d=0 in the formula and no work is done.