created meshes not affected by lights?

I'm finding that when I create a mesh using createmesh() createsurface(), addvertex, and addtriangle, the resulting mesh doesn't reapond to lights. say:

mesh = createmesh()
surf = createsurface(mesh)

AddVertex(surf, -1,-1,0)
AddVertex(surf, 0, 1, 0)
AddVertex(surf, 1, -1,0)

AddTriangle(surf, 0,1,2)

the triangle created by this segment of code doesn't respond to createlight() the same way that, say, a cube created by createcube() would. Is there a way to get it to respond to lights? or is there something simple that I'm doing wrong?
thanks

Try calling UpdateNormals once your mesh is constructed.

that'll do it. Thanks a lot