Air surrounds any object on all sides. Even if an object is resting on a surface there is always air (even if only a thin layer) underneath. The pressure of the air above will exert a downward force. The air below will exert an upward force. Explain and discuss how the combination of these forces results in a net buoyant force due to the object being immersed in a sea of air. Why does it work out that the resulting force has exactly the value that it does?
How would the situation be different if there were no air (a vaccum) underneath the object? Then, what would be the net force due to the air pressure? [Note: a situation like this can actually occur with certain precision blocks and surfaces used in machine shops. They are so extremely flat that if allowed to come into contact the thin layer of air that would normally be between them is excluded. What problem do you think allowing such surfaces to touch would produce?]