graphics - Animate point going around a triangle

How can I animate a point moving on a triangle's sides?
I can generate a triangle and point, but I have no idea how to define point movement. I'm guessing that I have to use $\sin$ and $\cos$ in coordinates, but I can't find a way to define the triangle in this way.
Can someone help me a bit with that?

Answer

If you interpret your geometric shape as a NURBS of degree 1 (linear), you can proceed with the following, extremely simple code:

pts = {{0, 0}, {1, 1}, {0.5, 1.5}}; (* just an example *)
s = BSplineFunction[pts, SplineClosed -> True, SplineDegree -> 1];
Animate[ParametricPlot[s[t], {t, 0, 1}, Epilog :> {Red, PointSize[Large], Point[s[t]]}], {t, 0., 1.}]

This yields the triangular (outer) graph of the following display:

Just replace Animate by Manipulate to give the user control over the point.

Note This is a rather general approach applicable in wide areas, since you can vary your control points as well as the spline degree, but the BSplineFunction will always yield the curve between arguments 0 and 1. In essence, you can display quite every geometric shape using this approach. For more complex ones, some adjustment to B-spline weights will be necessary, though.

The inner of those curves result from the same control points as before, but degree 2 and weights explicitly given as SplineWeights -> {.1, 1, 1}. Just exchange the s-line above with this one:

s = BSplineFunction[pts, SplineClosed -> True, SplineDegree -> 2, SplineWeights -> {.1, 1, 1}];

I hope this might be of some help to you.