Summarize:

You could try saying that you do clumsy stuff sometimes and you accidentally poked your eye which made it all watery. Probably most people have done this at some point or another, so this is something that could very believably have happened to you. Some allergies cause tears and puffy faces or eyes. You could say that you have an allergy that sometimes gives you these symptoms. To make it more believable, try following up with something about how it is for you to live with that allergy. For example, to keep the conversation light, you could say that it is super annoying to have allergies that make you look like a puffer fish. Sometimes when we get sick our eyes water. You could consider saying that you are getting over a cold that sometimes makes your eyes watery. You could try saying that your eyes dry up then tear up and are sensitive to gusts of wind or sudden changes in temperature. Maybe it was some dust or a bug or some eraser residue; whatever you decide to say, first look for something plausible in your environment you could have gotten in your eye and blame your tears on that. Keep in mind that whatever you do, don't lie and say that you got something dangerous in your eye, like a chemical. If you do this your teacher may rush you to the nurse, which is a waste of everyone's time. You will also cause people to needlessly worry and you may have to come clean about lying, which could get you into trouble. Sometimes we laugh so hard we cry. If you are wanting to hide your tears because you do not want someone to know that you were or are sad, and they weren't around you a minute or two beforehand, you could say that you were laughing about something really funny. Tell them a funny joke that you know or a funny situation that you encountered in the past to make it more convincing. Who knows, by recalling this funny situation, you may even cheer yourself up! Do a fake yawn by opening your mouth wide and breathing in heavily. Rub your eyes together and if someone asks, tell her that you get tears sometimes when you yawn. Whether or not it is actually true, some people think that our eyes water when we don't get enough sleep. If you want to hide your tears from someone who asks about them, tell her that you were up late last night doing homework or something else that you might plausibly have been doing the night before.
Say you poked your eye. Say you have bad allergies. Say you are fighting off a cold. Say you are sensitive to changes in the air. Say you got something in your eye. Say you were laughing hysterically earlier. Say you tear up when you yawn. Say you are short on sleep.