One man wanted to weigh the air. He blew a balloon and weighed it. Then he blew air out of the balloon and weighed again. The weight in both cases was the same. From this the man concluded that the air weighs nothing. Why is it wrong? Hint: What type of container should be Better used to measure the weight of air.

Because there's buoyancy force

Explanation:

What is wrong here is that he assumes that gravity is the only force affecting the weight of the balloon on the scale here. In reality there's also buoyancy force which lifts the full blown balloon upwards.

A better type of container would be an air tank where its volume does not change when it's empty and when it's full of air.