When 2-3 drops of detergent solution are added to a floating sewing needle, the surface tension of the water decreases significantly. This happens because detergent molecules disrupt the hydrogen bonds between water molecules, reducing the water's cohesive forces.

As the surface tension decreases, the buoyant force acting on the needle weakens. Buoyant force is the upward force exerted by a fluid (in this case, water) on an object submerged or floating in it. According to Archimedes' principle, the buoyant force is equal to the weight of the fluid displaced by the object.

With reduced surface tension, the water can no longer effectively support the weight of the needle, and it begins to sink. Detergent molecules essentially break the "skin" of the water's surface, causing the needle to lose buoyancy and submerge. The needle, which was previously floating due to surface tension, loses its ability to stay afloat and gradually sinks into the water.