take a balloon ,fill it with air and tie its mouth.
now take a straw and stick it to the balloon.
now open the mouth of the balloon.
we observe that the balloon moves in the forward direction by pushing the air backward.
This is an activity to demonstrate Newtons 3rd law.
You can also give an example of a person who is rowing a boat.
the driver pushes the water backward to move the boat in the forward direction