# Defogging a bathroom mirror

**URL:** <https://boards.straightdope.com/t/defogging-a-bathroom-mirror/27716>\
**Category:** Factual Questions\
**Created:** [August 1, 2000, 4:37pm UTC](https://boards.straightdope.com/t/defogging-a-bathroom-mirror/27716 "2000-08-01T16:37:03Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![bizerta](https://avatars.discourse-cdn.com/v4/letter/b/3be4f8/32.png) [@bizerta](https://boards.straightdope.com/u/bizerta)\
**Post date:** [August 1, 2000, 4:37pm UTC](https://boards.straightdope.com/t/defogging-a-bathroom-mirror/27716/1 "2000-08-01T16:37:03Z")

</div>

I’m trying to shave and my wife’s taking a shower and the mirror fogs. If I rub my hand over a soap bar and then rub my hand over the mirror, it no longer fogs. Why? The mirror is still cool, so I assume water would still condense on it. Does the soap create a layer of insulation? Does water still condense but the soap keeps it from creating droplets?

---

<div class="post-metadata">

**Author:** ![Ezstrete](https://avatars.discourse-cdn.com/v4/letter/e/7ea924/32.png) [@Ezstrete](https://boards.straightdope.com/u/Ezstrete)\
**Post date:** [August 1, 2000, 5:02pm UTC](https://boards.straightdope.com/t/defogging-a-bathroom-mirror/27716/2 "2000-08-01T17:02:44Z")

</div>

It’s probably because there’s glycerine in the soap.

Back before windshield defrosters, etc,one would rub a layer of glycerine in the inside of the windshield to prevent fogging.

Problem was that you then had to keep a piece of toweling handy to wipe up the moisture that accumulated at the bottom of the glass,where it met the dash.