Feeling (Vedana [vedanaa])* is like a "bubble of water"

"Now suppose that in the autumn ... when it's raining in fat, heavy drops ... a water bubble were to appear & disappear on the water, and a man with good eyesight were to see it, observe it, & appropriately examine it. To him ... seeing it, observing it, & appropriately examining it ... it would appear empty, void, without substance: for what substance would there be in a water bubble? In the same way, a monk sees, observes, & appropriately examines any feeling that is past, future, or present; internal or external; blatant or subtle; common or sublime; far or near. To him ... seeing it, observing it, & appropriately examining it ... it would appear empty, void, without substance: for what substance would there be in feeling? -Phena Sutta -Foam

v *Vedana [vedanaa]:

Feeling — pleasure (ease), pain (stress), or neither pleasure nor pain. See