How do you make gravy without lumps?

Chicken Gravy
Whisking together flour and water, and adding it to hot pan juices tends to produce lumps. Instead, make a roux, and add your hot water (or stock) to this, whisking consistency over a low flame. This should produce a lump-free gravy. If not, you may strain the gravy, and reheat It gently. If all else fails, and your gravy makes up in taste what it lacks in looks, keep the lights dim during dinner.
What is a roux?
A roux is a thick paste made of flour cooked in butter (to rid the flour of any chalky taste) or other fat, such as oil, lard, or beef drippings. Roux is used as a base for gravies and sauces, and as a thickening agent in dishes such as étoufée and gumbo.
While Northerners simply stir a little flour into butter to thicken their gravies, roux is taken very seriously down South, particularly in Cajun and Creole cookery, with some lard-based roux’s being cooked for an hour or more, until the mixture turns mahogany and the flavor becomes deeply roasted. This is called brown roux; mixed with beef stock, it becomes the base for brown sauce, which occasionally goes under the name Espagnole.
There is also white roux, barely cooked and suitable for light white sauces, and the longer-cooked blond roux, pale gold and slightly nutty, the choice for gumbo and other dishes that require a stronger flavor.