elo again guys. Well, my problem is that nite I got stuck with - which am sure is - a simple problem. Initially I wanted to simulate the Brownian motion in a cube. But then, I decide to try a simpler one using OpenGL. I want to make a point rebound to and fro along the same path. So, I guess I have to declare the function to do it as being a IdleFunc(); I declared a variable, GLfloat mol_x = -1.0; //this is the x-position of the point. then in the function, I wrote a contional statement as follows: if(mol_x >= -1.0 && mol_x < 1.0) mol_x += 1.0; else if(......) But I'm stuck with the else if part. I'm wonderingand I'm having doubts if this should be done in the IdelFunc or whether a recursive function should be used. Please help. Thanx.