If you haven't watered your Christmas tree since you forgot about it on Boxing Day, now is the time to get it out of your house. Unwatered Christmas trees are a huge fire hazard, as this video from the US National Fire Protection Association clearly shows.
Fires that start with a Christmas tree are also deadlier than the average house fire, according to a report by the same group. If you like keeping your tree around, though, keep it watered from the start. The second tree in the video was re-cut immediately before being put into its tree stand, and was watered routinely. It can still catch fire, but it's a slow burn rather than an instant conflagration.
Christmas Tree Fires Can Turn Devastating and Deadly Within Seconds [National Fire Protection Association]