What is the difference between an independent variable and a dependent variable in science?

My teacher told me that the difference between those two is that the dependent variable is dependent of the independent variable. I don't really get that. He tried to give us an example. If you stick an eraser up Mike's nose, his nose will bleed. He said the bleeding is dependent on the eraser going up his nose. I kind of got it after that but how do I DETECT what the independent and dependent variables are?|||think if you water plants with salt water... the amount that the plants grow depends on how much salt you use... so growth is dependent and salt is independent. Independent is the variable that you change on purpose. Dependant is just what happens