A weight lifter weights 100 Kg

A weight lifter weights 100 Kg and can lift 120 Kg of weight, so why can’t he carry himself and float in air for few seconds??

 

[Adrian Baker]

He can - it is called 'jumping'

 

[HallsofIvy]

Adrian Baker's answer was pretty good but I think what you are looking for is "for every action there there is an equal and opposite reaction". In order for a weight lifter to lift a weight, he has press down on the ground with an equal force- no ground, no force, no lift.