I suspect that this may be an exercise in tense shifting.
They watch how a kite flies - all in the present. They watched how a kite flew - tense shifted with the main verb. They watched how a kite flies - tense not shifted because this is an eternal truth (kites always fly in the same way).
With eternal truths, like the laws of science, there's no need to shift, and not doing so emphasises the eternal nature of the truth.