Often, you would want to try something out. Play around, experiment, but preferably without blowing things up or getting dirty. NumPy is perfect for experimentation. We will use NumPy to simulate a trading day, without actually losing money. Many people like to buy on the dip or, in other words, wait for the price of stocks to drop before buying. A variant of that is to wait for the price to drop a small percentage, say, 0.1 percent below the opening price of the day.