Recall from Lab 2 that the partial derivatives of a function f(x,y) at a point (x0,y0) can be found by looking the x- and y-slice curves of the function graph and then calculating the slopes of the tangent lines to these curves at (x0,y0,f(x0,y0). For the directional derivative, instead of slicing along the positive x- and y-directions, we slice the graph along a direction (cos θ, sin θ). The slope of the tangent line to the slice curve at (x0,y0, f(x0,y0) is the directional derivative. The demo allows you to choose a point (x0,y0) in the domain as well as a direction (cos θ, sin θ) using the two orange hotspots. The graph is shown in a separate window with the corresponding slice curve and slicing plane. The third window shows the slice curve in its plane along with the tangent line.