Momentum also can distort stock prices. Rising prices typically woo more buyers into the market, and the increased demand, in turn, drives prices higher still. Speculators often add to this upward pressure by purchasing shares in the expectation they will be able to sell them later to other buyers at even higher prices. Analysts describe a continuous rise in stock prices as a "bull" market. When speculative fever can no longer be sustained, prices start to fall. If enough investors become worried about falling prices, they may rush to sell their shares, adding to downward momentum. This is called a "bear" market.