What are the signs that indicate a rotting window sill and how can it be repaired or replaced effectively?

Signs of a rotting window sill include soft or spongy wood, discoloration, mold or mildew growth, and visible cracks or holes. To repair or replace a rotting window sill effectively, start by removing the damaged wood and cleaning the area thoroughly. Then, apply a wood hardener to strengthen the remaining wood and fill any cracks or holes with wood filler. Finally, sand the area smooth and repaint or seal the window sill to protect it from further damage. If the damage is extensive, it may be necessary to completely replace the window sill with a new one.