A soap bubble looks simple, but its wall is an extremely thin liquid film.
That film contains water held between soap molecules.
It cannot stay stable forever.
Gravity pulls liquid downward
The water in the bubble film slowly drains.
The upper part becomes thinner while more liquid moves toward the bottom.
The film therefore changes thickness throughout the bubble’s life.
That changing thickness also helps create the moving colors explained in why soap bubbles show rainbow colors.
Water evaporates too
At the same time, water is leaving the film through evaporation.
Dry air speeds that loss.
As the film contains less water, it becomes increasingly fragile.
One weak spot is enough
Eventually one small area becomes too thin or gets disturbed.
Dust.
A finger.
A dry surface.
A tiny defect.
Once a hole opens, surface tension pulls the remaining film apart extremely quickly.
The bubble disappears almost instantly.
Why does soap help?
Plain water does not make long-lasting bubbles easily.
Soap molecules stabilize the thin film and allow it to stretch.
That gives the bubble much more time than water alone would manage.
But soap cannot stop drainage and evaporation forever.
Why do bubbles last longer in humid air?
Higher humidity slows evaporation.
More moisture remains in the film for longer.
That can make bubbles noticeably more durable.
Can anything make a bubble permanent?
Not an ordinary soap bubble.
Its structure depends on a thin liquid film.
As long as gravity, evaporation, and disturbances keep working, the film will eventually fail.
Every bubble is temporary.
Some just get more time than others.
And that’s the short explanation.
Related explanations: Why do soap bubbles show rainbow colors?, Why does water evaporate?, and Why does glass fog up?