A group of test subjects are randomly assigned one of two treatments for their frequent headaches: generic or brand-name ibuprofen. Subjects taking the branded ibuprofen, Nurofen®, report markedly greater pain relief and fewer side effects than the generic group. The experiment is repeated with new subjects, but this time the Nurofen pills are replaced by Nurofen-labeled placebos. The result still holds. A sugar pill with a trademark can outperform genuine, but unbranded, pharmaceuticals. According to functionality doctrine, trademark protection cannot be granted for any feature that is essential to the product’s use or purpose, or affects the product’s cost or quality. But because of the placebo effect, even seemingly inert names and symbols are imbued with precisely this kind of power. In fact, a wide variety of real-world phenomena challenge the prevailing understanding of trademark functionality, from the social uses of high-fashion marks to the cost reductions enabled by certification marks. More fundamentally, a valuable trademark of any kind should act to reduce search costs for consumers and improve quality through the mechanism of reputation. And yet, rather than leading to invalidation, all of these well-documented functionalities are apparently tolerated by trademark law—sometimes merely ignored, but often celebrated explicitly. This article proposes a more unified theory of functionality: fragility. Some product features affect cost, quality, use, and purpose in ways that are non-fragile—the effects would persist even if every producer were to copy the same feature. But some features affect the product in ways that are fragile—the effects would be degraded or broken through unchecked copying. In reality, only non-fragile functionalities are actually prohibited, whereas fragile functionalities are permitted and even encouraged. In a manner surprisingly similar to patent or copyright law, trademark law appears to carefully distinguish between improvements that require its protection in order to manifest, and those that do not. This fragility theory is not only a descriptive improvement in terms of explaining real-world case outcomes and the doctrine’s full history, but also a conceptual improvement that can be applied to all types of trademarks without issue. A generic term, for example, exhibits non-fragile linguistic functionality. Moreover, the idea of fragility suggests consistent answers to divisive boundary issues in trademark law, such as overlapping protection under copyright, anti-dilution rights, and post-sale confusion doctrine. At the same time, recognizing this fragility pattern calls attention to potentially adverse consequences in terms of distributive justice and market competition—consequences that trademark law itself may not be able to remedy.