The Consumer Product Safety Commission posted a video that shows how fast your Christmas tree can turn into a FIREBALL if you forget to water it.
It shows two trees side-by-side. One's been watered every day, and the other hasn't. The one that HAS been watered starts smoldering, and there's a lot of smoke. But you'd have a decent chance of stopping it before your house went up.
Then they light the one that hasn't been watered and it's a raging inferno in about SIX SECONDS. You'd basically have NO chance of stopping it.
So if you got a real tree this year, make sure there's always water in the base. They can suck up as much as a gallon a day.