{
   TNtuple* nt = new TNtuple("nt", " for exercise", "x:y:z");
   TNtuple* nt2 = new TNtuple("nt2", " for exercise on correlation", "x:y:z");

   for(int i = 0; i<100000; i++) {
     float xtest = gRandom->Gaus(10, 2);
     float ytest = gRandom->Gaus(100, 20);
     float ztest = gRandom->Gaus(1, 0.2);
     
     nt->Fill(xtest, ytest, ztest);
     nt2->Fill(xtest, xtest*xtest, 10*xtest);
   }

   TFile result("nt.root", "recreate");
   nt->Write();
   nt2->Write();
   result.Close();
}