``After a while, the very act of stocks going up starts drawing in other people who get excited about the fact that their neighbours have made some money ... and that's when you get into the dangerous periods,'' he said.

Asked when the bubble will burst, Buffett said, ``You never know. You know that valuations are high, by historic standard. You know that the level of speculation is high, by any historic standards and you now that it doesn't go on forever ... but you don't know when it ends.'' Buffett told ABC's Ted Koppel that the ongoing market rally has ``gathered a lot of momentum,'' signaling that a downturn is aroundthe corner. Buffett told ABC that he has built a $30 billion endowment fund -- essentially his personal net worth -- for philanthropic purposes when he dies and said a team of six well-qualified people help determine how the money will be put to use. He said one area he was particularly concerned about was the increasing proliferation of nuclear weapons and technology and said he was funding some efforts to study the problem. But he acknowledged that he did not know how to ``attack the problem with money''.

He said his three children would receive some money from his estate after he died, but he did not believe in the ``divine right of the womb'' and felt they should prove themselves in society rather than inherit the great wealth he had amassed in his lifetime.