How to change the seed in gaussian distribution (boost)

I have found this example on How to use boost normal distribution classes?

#include <boost/random.hpp>
#include <boost/random/normal_distribution.hpp>

int main()
{
boost::mt19937 rng; // I don't seed it on purpouse (it's not relevant)

boost::normal_distribution<> nd(0.0, 1.0);

boost::variate_generator<boost::mt19937&, 
                       boost::normal_distribution<> > var_nor(rng, nd);

 int i = 0; for (; i < 10; ++i)
 {
           double d = var_nor();
  std::cout << d << std::endl;
 } 
 }

I was wondering what do I have do add to the code to generate a different ran dom numbers each time . I am using the number called d produced , in a for cicle , but at every time I run the programm I obtain always the same numbers. Thank you!!!!