system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do birds go forward when flying if their wing movement makes it look like they would only go upwards?
gpt
The flapping motion itself is generally used to build or maintain height. However, with the angle and shape of a birds wings, a bird can adjust its tilt slightly while flapping or gliding to build forward momentum. The forward momentum builds as the bird tilts forward, and will accumulate as the bird flies through the air. Birds can adjust the angle of their wings to flap with a combination of upwards and forward momentum.