2002 Honda Accord Engine burns oil

Hi, my car for some reason is burning oil. My car doesn't leak oil because I always park my car in the garage and there never has been a leak on the ground from it. But what I do noticed and realize that there is a problem and it's that I am always having to add 1 litre of oil about every 3 weeks just to keep the oil at level. When driving, my car doesn't show or make any signs of bad smoke from the back. So if it's not leaking oil, and not burning oil from the exhaust pipe? What would be the case why I am always having to add more oil so often because of the oil level being low? Thanks.