Blowing Soap Into Suds

You can make yourself a sort-of foam bath in the shower or tub this way with things you probably have in the bathroom already.  You'll need:
Get the washcloth thoroughly wet and evenly soapy.  Experience will teach you the proper ratio of soap to wetness.  Form a pouch with the cloth one of the following two ways so that it can completely cover your mouth:

Method #1: Hold the cloth in your non-dominant hand with thumb spread wide, and push with your dominant hand to form a pouch in the space between the thumb and forefinger of your non-dominant hand.  Then press the thumb and forefinger of your non-dominant hand around your jaw while its other fingers grasp the cloth tight under your chin, and your dominant hand grabs the remaining upper corner and pushes it to your cheek.

Method #2: Drape the cloth between thumb and forefingers of both hands.  Press these to your cheeks, pinching/bunching as necessary, while the remainder of the cloth hangs down/in front.
 
With a large washcloth, the effect is something like a feed bag.  The point is to have the cloth completely occlude to your face so no air can escape your mouth without going thru the cloth.  Method #1 keeps more of the soapy cloth away from your nose and eyes.

Inhale and apply the cloth as above. With the cloth in place, do not inhale further.  (Try not to cough, hiccup, or laugh, either, especially if using real soap (see below).  Real soap on the back of the throat can hurt for hours!)  Now blow thru your mouth.  When out of breath, remove the cloth and inhale again.  (You can try inhaling thru your nose with the cloth in place if using method #1, but real soap up your nose burns too.)  Re-wet and re-soap the cloth as needed.

Two factors influence the density of the foam produced by this method (assuming the cloth is wet & soapy to the max) -- the weave of the cloth, and the type of soap or other foaming agent.  If the cloth is not densely woven but is large enough, doubling it by folding it over neatly and blowing thru 2 layers will make a denser foam.
The densest foam is produced by using real soap, which usually comes in bars/cakes.  (Most liquid "soaps" these days contain no actual soap.)  Other foaming products -- shampoo, liquid "soaps", shower/bath gels, body washes, hand dishwashing ("washing-up" to the British) detergents -- make a looser foam of larger bubbles.  The tradeoff is irritancy.  Having real soap pressed against your chin and around your lips can make your skin sore after a while.  You may want to protect your skin there with a silicone barrier cream before doing this.  The other detergents mentioned above, even most of those for washing dishes and utensils by hand, are milder than real soap in this use.

Among soaps, the best to use for this are the most lathery -- i.e. those made with from most tropical oil (coconut, palm kernel/seed, or babassu) and the least tallow, olive, or palm oil.  That'd make Kirk's Coco Castile the strongest, followed by Rainbow Research, then Camay, then a few that are probably about equal -- the new Dial, Coast, Lux, Lifebuoy, Shield -- then Ivory Soap, then many others.  At the opposite extreme would be Pure & Natural and finally Neutrogena. Unfortunately the sudsiest are also the most irritating.
 
A pretty good compromise are Olay and the discontinued Ivory Moisture Care bars.  These almost-soapless detergent cakes don't make quite the density of lather that real soap does, but they do have a creamy quality (enhanced by paraffin wax) that compensates somewhat.  You can blow all you want and probably never get sore around the mouth.  Almost the same (even though containing a lot of soap) are Lever's Dove and Caress bars, at least in the USA version.&nsbp; The suds of all these last fairly long even on top of hard water, and leave no ring.  However, the Olay bars do tend to leave a coat of wax on the surface where the soap dish drains.

If your water's soft (or you use Olay, Dove, or Caress) and you work at it, you can cover your bath water with bubbles this way.  Even if your water's not so soft, you can at least pile the suds on yourself or your kids, where they'll last until the water hits them.  And you can do it in the shower, too.

I discovered this method as a teenager in the shower.  It was a modification of a method my sister and I used to use in the bathtub, where we'd trap air under the soapy washcloth and either squeeze it out or drag it under.

Am I worried that dissemination of this technique will reduce demand for my invention? Not really; most grownups I've tried to teach this to don't want to do the work required, let alone teach it to their kids.  However, I'm hoping to sell this trick to photographers for glamor shots and soap commercials.