Well that's quite simple my dear. Let me ask you a question: Have you ever seen a hummingbird with their long beak in a flower? Well that's because the hummingbird was eating from the flower! Flowers produce a sweet liquid called nectar. This is a hummingbird's food. Okay so now you know what the flower does for the hummingbird. But what does the hummingbird do? Well have you ever wondered how flowers seem to have offspring that grow even though they can't move? Well you see, when the hummingbird drinks from the flower, the plant releases small seeds onto the tiny bird. Some of these seeds cling onto the bird's feathers. These seeds move around with the bird. Eventually the seeds become detached from the hummingbird and fall onto the ground. They then get buried into the soil, begin to grow, and the process starts all over again! So to summarize what I just said, the flower helps the hummingbird by providing it food, and the hummingbird helps the flower by carrying its seeds around, helping it to reproduce. I hope this helped! Oh and by the way, if you could post questions like these in the Science section instead of the English section I'd really appreciate it!