How often do you use excuses to cover up for something? How often do you fail miserably? A lot, right? That's where I come in. See, I am the master of excuses. I rule in making up excuses. So, being the nice guy that I am, I'm going to share my knowledge with you all. Use it with care, young ones.
The golden rule of excuses: If you wouldn't believe your excuse, others won't either. You're not that smart.

First off, here's some rules you should follow when creating homework excuses. Others will follow soon.

1: Don't use cliches.
One of the things most people do wrong when making up an excuse, is that they say whatever comes up in them first. Whatever comes up in them first is usually something that they hear a lot. So, they end up with cliched excuses. Remember, there are more lazy students out there, and your teacher has other students as well. So she has probably already heard of homework assignments being eaten by pets several times that day. Which brings us to our next point....

2: Pets rarely eat homework.
Yup, you heard it right. I mean, how often has your dog actually tried to eat your homework, or some other piece of paper? Now, if your homework assignment was a shoe, this could very well be the case, but dogs, cats, canaries or other pets rarely ever eat homework, so this one won't work kids. Unless you have a goat for a pet. Then it might work. It's well known that goats eat almost everything, so if you happen to live on a barn, or just like goats a lot, go for it!
A better idea for the pet-eating-homework excuse would be making a more plausible variation to the excuse. Like "My cat shitted all over my homework". This may gross your teacher out, but that could be a good thing.

3: I didn't know!
This one rarely works, unless well prepared. See, even if you didn't know you had homework to make, you could've asked some one else wether you did. That's the way teachers work: in their little fantasy world, when someone thinks that he/she has no homework, they always double check by asking someone else. In reality, this doesn't work, but they don't know that. So a good "I didn't know" excuse always needs someone to back you up. Do make sure that your backup has made their homework his/her homework, or this won't work. Then, make that person say something like, "Yes, he/she asked me, but I had already finished my assignment a while back, so I forgot."
If you can get a nerd to do this for you, it would be perfect, because teachers LOVE nerds (see appendix). But nerds will rarely grant you a favour, so you'll have to turn to your friends.

4: I forgot my books!
This one might actually work, but only with certain teachers, and not too often. Most teachers will make you pay dearly for "forgetting" your book, so you could as well say that you didn't make your homework. An exception is when you have several books of diffrent classes that look very similair, or even better, multiple books for the same class. Then you can say you brought the wrong books. Still, this one stays kind of a gamble, so use at your own risk.

Appendix:
Teachers hate students. It's a fact. They hate you and will do anything to screw you over. Except if you're a nerd, then they love you. This is because of a very simple reason: teachers used to be nerds themselves.See, there is this vicious circle you have to know by heart to survive school: Teachers used to be nerds, so if you're normal in any way (like, if you actually have a social live), they won't like you because you remind them of the kids who used to pick on them. The nerds in the class however, remind thet teachers of how they used to be themselves, so they have it good in class. The nerds like the teacher as well because of this, and they see that the teacher has power over all the other "normal" kids, so they want to become teachers themselves. The circle is complete.