So in diagram A before we added our detergent, we had a relatively high surface tension. Now when we add our detergent, we decrease the surface tension of that liquid. The question is why? Well, to begin, let's actually define what surface tension is. So surface tension basically means that the molecules found on the surface of the liquid remain on the surface and they're able to bond very well with adjacent liquid molecules. In the case of water, it's adjacent water molecules.