3-legged chair to teach proper musical posture

When in any kind of band or music class, the first thing they ever teach you is posture. "Always keep your bum on the edge of your chair and sit up straight." My band director is always yelling at us about our horrid posture.

<true story>This morning, I was practicing, and remembered one of my my desk chair's back legs had broken, leaving it three-legged. In my infinite boredom, I decided to try to sit down on it, and alas! I discovered that it forced me into proper musical position!</true story>

So, make chairs with one collapsible back leg. On the chair, normally the back leg itself is strong enough to hold up the chair, but once someone sits on it, the back leg collapses and gives the musician a startling surprise.

Yes, Public:Evil is the correct category for this.