The face of a penny can hold about thirty drops of water.
A Penny Can Hold 25 Drops of Water on Its Surface
If you've never tried the penny-and-water experiment, prepare to have your mind blown. Take an ordinary penny, an eyedropper, and some water, and you can stack about 25 drops of water on that tiny copper surface before it finally spills over the edge. Not five drops. Not ten. Twenty-five.
The secret? Surface tension—one of water's most fascinating properties.
Why Water Clings Together
Water molecules are incredibly clingy. They stick to each other through a force called cohesion, creating an invisible "skin" at the water's surface. This skin is what we call surface tension, and it's strong enough to let insects walk on water and allow you to build a dome of liquid on a penny.
As you add drops one by one, the water doesn't just spread out and fall off. Instead, it piles up, forming a bulging dome that rises above the penny's edge. The cohesive forces between water molecules are holding that dome together, fighting against gravity with impressive strength.
The Experiment in Action
Scientists and teachers use this as a classic demonstration of surface tension. Here's what happens:
- Place a clean penny on a flat surface
- Use an eyedropper to carefully add water drops to the center
- Watch as the water builds upward into a dome shape
- Count the drops—most people get between 23 and 27
- Eventually gravity wins and water spills over the edge
The exact number varies based on dropper size, how clean the penny is, and your technique, but the result is always surprisingly high.
Break the Tension
Want to see surface tension in action even more clearly? Try the experiment again with soapy water. Soap is a surfactant—it breaks down surface tension. With soapy water, you'll only get about 12-15 drops before the dome collapses. That's nearly half as many as plain water.
This dramatic difference shows just how powerful water's natural surface tension really is. Soap molecules wedge themselves between water molecules, weakening their grip on each other and making the "skin" fragile.
Surface Tension Everywhere
This isn't just a party trick. Surface tension is working all around you:
- Water striders and other insects walk on ponds without sinking
- Water beads up on a freshly waxed car
- Raindrops form spherical shapes as they fall
- Capillary action pulls water up plant stems
The penny experiment just makes this invisible force visible. Those 25 drops stacked impossibly high are proof that water molecules are holding hands tighter than you'd ever expect.