Little Johnny failed his math test. When he returned home, his father was standing crossly in the doorway.

“Why did you fail your test?” he asked.

Little Johnny shrugged and said: “On Monday, my teacher said 3+5 = 8.”

“So?” his father answered.

“On Tuesday, she said 4+4 = 8, and on Wednesday she said 6+2 = 8. If she can’t make up her mind, how am I supposed to know the right answer?”