There are two blocks on the single rod. Lift the top solid with the help of your single hand, until it reaches the stopper and bring it down to its original position. Now, hold the bottom solid with your single hand and lift both the solids together.

What did you observe?
It is easy to lift both the weights than lifting the top one alone.

The science behind it :
The preception of our mind is demonstrated well with this fun filled gadgets
When you are about to lift both, your mind assumes the energy spent for the top and exerts almost double the energy to lift both. Since the weight of both blocks together is less compared to the weight of top part doubled, it becomes easy for us to lift both.