1. They can flow: Fluids can change shape easily and continuously under the influence of shear stress. This means they can move around obstacles and take the shape of their container.

2. They are incompressible: Fluids resist changes in volume under pressure. This means that their density remains relatively constant even when subjected to pressure changes.

It's important to note that while these are general characteristics, there are exceptions. Some fluids can be compressible under extreme pressure, and some may exhibit "non-Newtonian" behavior where their viscosity changes based on the applied stress.