A coin. A dropper. And a question nobody can answer correctly before they try: how many drops of water can you fit on the surface before it spills?

The answer is almost always more than anyone expects — sometimes 30, sometimes 40. And the reason is the same invisible force holding every raindrop, every dewdrop, and every soap bubble together: surface tension.

Water molecules are attracted to each other. At the surface, there are no water molecules above — only air — so the surface molecules pull each other sideways and downward with extra force. 

That pulling creates a tight, slightly elastic skin. It's the same skin that lets a paper clip float if you lower it gently onto still water, and the same skin that pond skaters sprint across without sinking.

Each drop you add stretches that skin a little further. 

You can watch the dome grow — water swelling above the rim of the coin, holding itself together through pure molecular attraction. The moment the weight becomes too much for the skin to bear, the whole thing collapses at once.

Did you notice the dome grew much larger than expected before it burst? And that it went suddenly rather than gradually? That's the skin holding, holding, holding — then giving all at once.

To see what happens when soap destroys that skin in an instant, try [Magic Milk Reaction]. And to see the same cohesion between water molecules doing something completely different, [The Colour Climb] shows water pulling itself upward through a paper towel using the very same attraction.

But here's the question — does water temperature change how many drops you can fit? Cold water and warm water behave very differently.

 Find out in The Crazy Scientist Lab.