Super smart! The red part occupies a much larger area than the seeds. So, most marbles that hit the watermelon will actually hit the red part of the watermelon.
The red part is supposed to be like the protons in an atom. Also, recall that if you throw one positive charge at another positive charge, the two positive charges repel each other.

Now, instead of throwing marbles, imagine throwing some positive charge at a thin metal sheet. The sheet is made of atoms. So, just like the watermelon, the metal sheet has a lot of protons (positive charge) spread around, and a few electrons (negative charge).
What will happen to the positive charges that you throw at the metal sheet?
  • Most charges will pass through
  • Most charges will return
  • Exactly half will go through, half will return