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Instructions.

Press “Run the isotherm” to send the runner around the level curve $x^2+3y^2=4\text{,}$ or “Dash for cool air” to send her along the path of steepest cooling. The readout shows her temperature, position, directional derivative $D_{\mathbf u}T\text{,}$ and distance travelled; the blue arrow is $\nabla T$ at her position. Use “Pace” to change her speed, and “Pause” or “Reset” to control the animation.