[ODE] please help me about ODE!

[=?gb2312?B?zfW41A==?=]

I write code as below, and expecting the collision, but nothing happened, who can tell me why?

thanks a lot

//*************************************************************
 dSpaceID RootSpace = dSimpleSpaceCreate(0);

 dGeomID GeomID = dCreateSphere(0, 2.0f);

 dGeomID t1 = dCreateGeomTransform(RootSpace);
 dGeomTransformSetGeom(t1, GeomID);
 dGeomSetPosition(t1, 0, 0, 0);

 dGeomID t2 = dCreateGeomTransform(RootSpace);
 dGeomTransformSetGeom(t2, GeomID);
 dGeomSetPosition(t2, 0.5f, 0, 0);

 const int N = 2;
 dContactGeom contactGeom[N];
 int n = dCollide(t1, t2, N, contactGeom, sizeof(contactGeom));
 if ( n > 0 )
 {
  //do something if collision;
 }

 dSpaceDestroy(RootSpace);

//*************************************************************