Pure water has a surface tension of about 72-73 mN/m at room temperature (72.8 mN/m at 20°C, ~72.0 mN/m at 25°C) — one of the highest values among common liquids, second only to mercury. This comes down to hydrogen bonding: every water molecule clings tightly to its neighbors, and the molecules right at the surface — which have fewer neighbors pulling on them — form that elastic "skin" that lets, say, a metal paperclip float on water.
Why plain water can't make bubbles
For a liquid film to survive as a bubble, it needs to be elastic enough to self-heal when stretched (the Marangoni-Gibbs effect) and thin enough not to snap on contact. At 72 mN/m, pure water just can't pull it off: any film thins out and gets yanked apart by its own tension before it can stabilize. That's why a wand dipped only in water never produces a real bubble — just a drop that falls off.
The fix is a surfactant (soap): its amphiphilic molecules park themselves at the surface and drag tension down into the 20-30 mN/m range, giving the thin film enough time to exist. In the KUANTIKA engine, this reference value (72 mN/m) is the baseline against which we measure how hard each formula "works" to bring tension down using soap, polymer, and humidity. See also air-water interfacial tension and how soap lowers surface tension.