# Why is my toothpaste scared of water, or is it the other way round?

**URL:** <https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399>\
**Category:** Factual Questions\
**Created:** [May 16, 2002, 3:43pm UTC](https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399 "2002-05-16T15:43:29Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Jukeball](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@Jukeball](https://boards.straightdope.com/u/Jukeball)\
**Post date:** [May 16, 2002, 3:43pm UTC](https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399/1 "2002-05-16T15:43:29Z")

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Ok so I’m brushing my teeth , with all the froth in my mouth , my basin wet with water , when all of a sudden a drop of froth falls in the basin and the water layer around the fresh froth in the basin runs away from it!!! and the froth contracts somewhat , but mainly its the water running away!  
Why does this happen?

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**Author:** ![Jukeball](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@Jukeball](https://boards.straightdope.com/u/Jukeball)\
**Post date:** [May 16, 2002, 3:44pm UTC](https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399/2 "2002-05-16T15:44:47Z")

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mistake , it supposed to be my toothpaste not my tooth! Sorry. If the mod could correct it in the subject

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**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\
**Post date:** [May 16, 2002, 8:11pm UTC](https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399/3 "2002-05-16T20:11:05Z")

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> [@](#):
>
> \*Originally posted by \*\*\*Afghan \*\*
> 
> mistake , it supposed to be my toothpaste not my tooth! Sorry. If the mod could correct it in the subject

Done.

**DrMatrix** - General Questions Moderator

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**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\
**Post date:** [May 16, 2002, 9:14pm UTC](https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399/4 "2002-05-16T21:14:38Z")

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The main thing is that toothpaste contains a surfactant (usually sodium lauryl sulfate). Surfactants act upon surface tension of water. On a hydrophobic surface, water will “bead”; when you add a surfactant, the surface will get wet.

Your glazed sink is a fairly hydrophobic surface, and the water on it is beaded at the edges (one big honkin’ bead, in a sense). When you drop in a bit of toothpaste, the water doesn’t :“run away”; it actually “rushes in” and wets the surface. What you’re seeing is the new edge of the bead, which only forms on a surface that’s clear of surfactant.

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**Author:** ![Opengrave](https://avatars.discourse-cdn.com/v4/letter/o/919ad9/32.png) [@Opengrave](https://boards.straightdope.com/u/Opengrave)\
**Post date:** [May 17, 2002, 8:31pm UTC](https://boards.straightdope.com/t/why-is-my-toothpaste-scared-of-water-or-is-it-the-other-way-round/109399/5 "2002-05-17T20:31:11Z")

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\<Mr. Wizard\> Try this cool experiment, take a bowl and fill it mostly full with water. Sprinkle a good amount of ground pepper on the surface and (before the pepper starts to sink) touch the corner of a bar of soap in the center of bowl of water. Watch as the pepper ‘runs away’ from the spot where the soap entered the water. \</Mr. Wizard\> Not rocket science but my four year old thought I was _the coolest_.

NP: **Dark Tranquility** - _Projector_