"Vitamins are like tire irons. Without a tire iron, you can't
change a tire. A box of tire irons won't help you change it faster.
This is a good way to explain that excessive vitamins don't help,
and may do harm."
The link is a paper called "Essential Analogies in Human Anatomy & Physiology" It has many analogies used for teaching.
Feel free to be the first
Please review the linked page for context.
If you can think of something better than this,
please add it to the database