When a painting brush is dipped in water after painting then Fibres of brush spread out but as soon as it taken out from water the Fibres cling together . Because When the brush is dipped in water, the bristles are lifted up by the buoyancy of water. As a result the bristles drift apart. When the brush is taken out from the water, it is wet. Adhesive force between the molecules of water causes the bristles to cling together.
Picture Credit : Google
