It's a lot of work to find the right stock to add to your portfolio. Do you need a growth stock, a value play, or simply a high yielder to generate income? It's great to finally find the right investment that meets your specific needs. In the case of master limited partnerships, however, it's important to make sure the stock you're buying passes one very important test. In this video, Fool.com contributor Aimee Duffy talks to Tyler Crowe about the levels of liquidity in MLPs such as CVR Refining (NYSE: CVRR), Northern Tier Energy (NYSE: NTI), Phillips 66 Partners (NYSE: PSXP), Kinder Morgan Energy Partners (NYSE: KMP), and more. Aimee explains why it is crucial for MLP investors to factor it in to their investment thesis before buying.