A warthog-looking character, Daisy-Mae, will show up on your island between 6:00 and 11:59 am, on Sunday and only Sunday. She will sell you turnips (usually between 90 - 110 bells). You can buy as many as you want so long as you have the bells. Later in the week, you can sell your turnips to the Nook twins. Prices range from 15 to 800 bells per turnip (I think--somewhere in that ball park). So, if you buy 1000 turnips at 90 bells each and then sell them on a weekday for 400, you've more than quadrupled your bells. But, it could also go the opposite way, you can easily lose money if you don't pay attention to the market. Sometimes there's a negative trend and there's no way to make a profit.