Take an empty plastic bottle . Close the mouth of the bottle with an airtight stopper . Put it in a bucket filled with watere . The bottle floats .
Push the bottle into the water. You feel an upward push. Try to push it further down. You will find it difficult to push deeper and deeper. Water exerts a force on the bottle in upward direction. Now release the bottle. It bounces back to the surface.
How can you immerse the bottle in water ?

Explanation

If downward force mg is greater than the upward force of water, then the bottle will sink. If we fill the bottle with sand and put it in water, it will sink. Here, mg is greater than the upward force of water.