Their brains are going through important changes. Literally the axons and dendrites are being "clipped out". The hedges are being trimmed to make way for new neuronal connections.
They cannot think very well during this time and tend to be very forgetful. When asked why they did something the answer will always be the same, "I dunno!" They truly don't know!
Don't be harsh on them, love them through it and use TONS of sticky notes!!!
Also the hormonal changes are affecting their emotions too. Boys think only of sex sex sex and girls worry about how everyone sees them.
It is a rough time for the parents and the teens too. The pressure is on. They are becoming adults, they are torn between loving their parents and listening to their friends' usually bad advice. They know they will have to move out in a few years, it freaks them out. They are afraid to lose their parents, so they push them away.