Here's 3 ways that you can avoid getting in trouble...
#1 - Just tell the teacher the truth
Instead of just making something up that you know that your teacher would never believe anyway, just go ahead and tell them that you honestly forgot, that you have no excuses, that you're sorry, and ask for an extension. I can almost guarantee that they'd be happier than you giving them some phony excuse.
#2 - Explain to your teach that you tried but didn't really understand
As long as you show that you attempted to do your homework, the teacher should understand... well... hopefully...
Then ask them if they can help you go over some of the questions when they have time. This will actually help you learn a little more as well as possibly get brownie point and an extension.
#3 - If both of the above are epic fails then make your excuse worth it!
I mean, if you're gonna get in trouble anyway, then why not go out with a bang. Don't use the lame excuse of "my dog ate my homework". Instead go for something completely unbelievable that the teacher will hopefully just send you on your way and forget about it all together. "You see, I started to do my homework and then all of a sudden there was a huge bang outside. This is when I saw the UFO that had crash landed in our huge tree in front of the house. There was no time to call 911, plus my phone had just been eaten by a zombie, so I spent the entire evening trying to help the aliens get out of the UFO and climb down the tree. They weren't great climbers, so it took a really, really long time. So you see, there was really no way I could even complete my assignment because I was busy saving lives!".