An interesting way to explore and explain Newtons 3rd Law: If birds in a truck fly, does the truck get lighter?
My brother explained it as follows: You’ve got molecules of air in between the birds wings and the truck. The only way the birds can push off the ‘ground’ (ie. fly) is by producing enough force on these molecules to create sufficient lift. This force/lift is proportional to the mass of the birds because gravity is constant. This force/lift acts on the body of the truck. In other words, the lift caused by the bird’s wings exerts a downward force on the truck. This force is equal to their (the birds) mass… times acceleration due to gravity.
However, it depends on whether the truck is closed. If not, some of the force caused by the bird’s wings can go off the truck. If it was not, imagine you are walking under a bunch of birds and you should feel they are pushing you down.