Soon Tom said that they were interfering with each other, and neither getting the fullest benefit of the tick.

So he put Joe's slate on the desk and drew a line down the middle of it from top to bottom.
"Now," said he, "as long as he is on your side you can stir him up and I'll let him alone; but if you let him get away and get on my side, you're to leave him alone as long as I can keep him from crossing over." "All right, go ahead; start him up." The tick escaped from Tom, presently, and crossed the equator.

Joe harassed him awhile, and then he got away and crossed back again.

This change of base occurred often.

While one boy was worrying the tick with absorbing interest, the other would look on with interest as strong, the two heads bowed together over the slate, and the two souls dead to all things else.