The problem: you're swimming across a river. You can swim at a fixed speed, in whatever direction you choose. But the river current is also pushing you sideways the entire time you're in the water. Where you aim and where you actually land are usually two different places — the slider below controls which direction you aim.
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Which way are you aiming? Straight across
← aim upstreamaim downstream →
Your swimming speed 1.5 m/s
River current speed 1.0 m/s
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The gap between "where you aimed" and "where you landed" has a simple explanation: the current pushes you sideways for the entire time you're in the water. Drift = current speed × time in the water. Swim faster or cross at a steeper angle, and you spend less time in the water — so the current has less time to push you off course.