{
  TFile *hfile = new TFile("junk.root","RECREATE");
  ntuple = new TNtuple("ntuple","Demo ntuple","px:py:pz:random:i",2000);
  hfile->Write();
  Float_t px, py, pz;
  for (Int_t i = 0; i < 25000; i++) {
     gRandom->Rannor(px,py);
     pz = px*px + py*py;
     Float_t random = gRandom->Rndm(1);
     ntuple->Fill(px,py,pz,random,i);
  }  
}