Find a rock with deep water below it that is safe to jump into.
Stand on the rock and feel its solidity beneath the soles of your feet. Contrast that feeling with the swirling water before you. Explore the rock with your hands. Feel its strength and test its handholds.
Now look at the rock. See the marks on it, signs of erosion; the rock is slowly dissolving. See the cracks in it; the rock is crumbling.
Think about how the rock you're standing on may have come into existence. If it's limestone, for example, it was made from the compressed exoskeletons of countless tiny marine creatures that lived long ago.
The rock was not always there and in the future it will disappear. Can you be truly happy, placing your faith in impermanent things – health, relationships, wealth, work, beliefs – when deep down you know the truth? It is in the nature of all these things to decay or change.
Now, let go of the rock, and all false sources of security, and jump in! Feel the restless water caressing every part of you, leaving no spot untouched. As one water molecule drifts off another arrives to take its place. There is nowhere to stand and nothing to hold onto, yet the water holds you up!