On its own, ice is not slippery. When you step onto an icy sidewalk, you do indeed feel a slippery surface. But the slipperiness is caused by a thin layer of liquid water and not directly by the solid ice itself. Water on a smooth surface is slippery because water is a low-viscosity liquid. As such, there are no permanent intermolecular bonds in liquid water, and the transient intermolecular bonds are weak. This means that water molecules can move about freely, slide past each other easily, and fill any microscopic holes or cracks that would snag an object.