# Why does water flow off the penny only after multiple drops? How come that the penny « catches » it?

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In: Physics

Water has really high surface tension. Basically water likes to stick to itself and it takes a bit of effort/force to make it separate.

So what you are seeing is basically the water molecules all holding hands to stick together until there are just too many of them and their handholds break.

It’s not the penny that catches it, it’s actually the water itself. Liquids have surface tension, which holds the water bubble together like a balloon holds air.

You can see at certain points where the water bubble actually extends over the side of the penny, but doesn’t fall down. That’s surface tension holding the water together.

Eventually as more droplets are added, the water becomes too much for the surface tension to hold and the bubble breaks, similar to a balloon bursting. Everything rushes out and flows away.

Essentially, water sticks to itself. That’s why water forms into droplets or puddles instead of, say, a fine mist. This quality is called cohesion. Water is highly cohesive.

That cohesion keeps it from flowing off of the penny, because it doesn’t want to separate. Eventually, when enough water is hanging off the edge of the penny, it gets too heavy to support itself, and breaks off. When that water breaks off it lands on the ground, and would normally fall away as a droplet.

BUT, because the penny is so thin, the water droplet that breaks off on to the table is thick enough to still be touching the water on the penny.

Now the water on the penny is cohesive to itself, and also the water on the table, because they are both water

The water is only cohesive to water, not to the penny, and since gravity is pulling the water down, and since it is also cohesive to the water on the table/ground, it travels to the table.

Essentially, it’s like a bunch of bling monkeys who always try to stay together. As you put them on the platform one by one, they try to hold onto each other to avoid falling off. Eventually there’s just too many on the platform, and one falls off. When the monkey who falls off lands just a few feet below, the rest realize that they don’t have to stay on the platform, and they just leave.