I'm going to play devil's advocate here and try to prove that the earth is flat (even though I don't believe it)...
Ok, to prove the earth is flat, grab a big ball of clay.
Roll the ball of clay into a nice round ball
Take your fingers and push in large areas of your clay ball so you have areas that are indented.
Now take another mound of clay and flatten it out like a pancake.
Use your fingers to make indented areas like you did on your clay ball.
That clay ball represents a round earth and the flat clay represents a flat earth. The indented areas represent the oceans and lakes.
Now take a glass of water and carefully pour water into your oceans and lakes..
As you can see, the water just falls out of the round ball because it's round. Therefore, since the real oceans are still full of water, the earth has to be flat.