There are tentative signs that BP's latest tactic, known as 'top kill,' to plug the oil leak in the Gulf may actually be working. The process involves "injecting heavy drilling fluid deep into the well in hopes of stemming the relentless flow of gas and oil."
This is obviously a complicated process, and one that has never before been tried underwater at this depth. The chemical reaction at work is "dilatancy."
For an explanation of this in terms the layman can understand (along with an interesting visual aid), we turn to Bill Nye (The Science Guy.)