1
The first thing that comes up is that people have to decide how to make their batch of foam. Often, people will guess that the more soap they add, the stronger the suds will be

But this ends up with foam so heavy that it oozes off the plate before it can build up any height!

At this point, a group with foam that was too thick might need to start over. Simply adding more water or soap to the batch is unlikely to come up with a mixture that would work.

2
Once a group has a mixture they are able to build with, they are likely to need to carefully sculpt their foam into a tower shape.

3
People may realize that they need light foam on top, and are left with their heavier foam at the bottom of their bowl. As they look around to see if anyone else has light foam, they may see a waste bucket full of foam from the previous group. This will have very light foam on top, which they may ask if they can use.

You have to decide if you want them to be able to simply scoop this easily stackable foam up, or if you want to ask them how they think the foam got that way and if they can make it themselves.

Some people will cheat (this person is piling foam on paper cups). They almost always know they are cheating but are ususally frustrated, or simply curious if they can meet the challenge even by using other materials.

4
Other people, usually the patient ones, succeed!