Have you ever chewed bubble gum, then you blow air into it. The bubblegum makes a round formation. If you blow air too quick or the round formation becomes too large thus have weak walls and bursts? That is a bubble, that in turn burst. Markets behave in a similar fashion. Bubbles are normal. Any market goes through these cycles. Much like my bubblegum scenario, you never know when the bubble will burst. Yes, there are signs but noone knows for sure.