Since the 1930s, liquid shampoo has shaped expectations around foam, detangling, and softness. Creating a high-performance solid shampoo is a real technical challenge. To adopt solid shampoo sustainably, reconsidering what is expected from shampoo may be necessary. Washing hair is perceived as simple, yet modern shampoos are expected to remove excess sebum and dirt, foam sufficiently, spread and rinse quickly, and leave hair shiny, easy to detangle, and pleasant to the touch. Additional requirements include reducing packaging, particularly plastics, and reducing the environmental impact of the product. Solid shampoo appears to offer responses to some of these concerns, but after an initial surge in popularity, progress has stalled. Before the 20th century, soap was used for washing hair, but its alkaline nature caused hair to swell and change its surface. In the presence of calcium- or magnesium-rich water, soap could form insoluble deposits, leading to dull, rough, and hard-to-comb hair. These drawbacks led to the development of synthetic surfactants, known as syndets, which work at pH levels better suited to hair and scalp. Starting in 1930, the first soap-free shampoos were marketed in Europe and the United States. Over the decades, shampoo compositions have become more sophisticated, incorporating different surfactants to adjust cleaning and foam, conditioning agents to facilitate detangling, and other ingredients to affect viscosity, stability, and sensory properties. These innovations transformed shampoo into a care product and a pleasure to use. In the mid-20th century, advertising campaigns in France promoted hair hygiene, emphasizing shine, foam, and the pleasure of washing. The Foam Competition was a radio game that highlighted the increasing importance of foam in the representation of shampoo. Foam contributes to product distribution but does not directly measure cleaning effectiveness. It plays a significant role in the perception of performance and the user experience, with recent studies showing that the experience can have an emotional dimension, even influenced by the sound of foam. Other properties, such as ease of rinsing, detangling, and the feel of hair after drying, also contribute to the user experience. These once novel performances have become quality criteria that are now considered standard. The rise of solid shampoo aligns with a broader movement in the cosmetics industry, with deodorants, facial cleansers, toothpastes, and after-shampoos now available in solid forms. Solid shampoo can significantly reduce the amount of product needed per use and packaging, particularly plastic. It also allows for a reduction in environmental impacts related to transport and packaging. The low water content changes microbiological constraints, with a recent survey showing that preservatives are less important in solid shampoos than in liquid ones. The image of simplicity and naturalness associated with solid formats is often linked to the format itself, but these characteristics alone do not guarantee a better environmental balance across the entire life cycle. For instance, the use phase, particularly the hot water consumed under the shower, can represent a significant part of the impacts. Despite this, the movement has been rapid, with the proportion of consumers purchasing solid cosmetics in France increasing from 1% in 2019 to 7.5% in 2021, and the market reaching 85.5 million euros between November 2021 and October 2022. The return to a solid form is not a return to soap. Many solid shampoos rely on surfactants other than soap, known as solid syndets. "Synthetic" does not necessarily mean "derived from petroleum," as some surfactants are obtained from renewable sources such as fats, sugars, or amino acids. However, the transition to solid shampoo requires completely reformulating, not just concentrating an existing recipe. Difficulties in solid formulations concern the hardness and plasticity of the bar, its cracking, softening, loss of mass during use, and the quality of the foam obtained. The bar must be cohesive enough to be manufactured, transported, and handled, resistant enough to not dissolve quickly in a humid bathroom, but sufficiently accessible to water to effectively deliver its components during use. The need to regain the softness and detangling that liquid shampoos have accustomed users to adds to these challenges. Replacing conventional ingredients with alternatives of natural or bio-sourced origin also poses performance, stability, and compatibility challenges with formulations. Improving one property can degrade another, requiring a search for the best compromise between performance, sensory properties, and new sustainability requirements. The technical difficulties are compounded by behavioral challenges. After an initial rise in the early 2020s, the French market for solid cosmetics reduced by 3.2% in volume in 2022 and 2023. This decline may not be solely attributed to shampoo, as factors such as price, distribution, economic context, and product evolution can influence market trends. It highlights that initial enthusiasm does not necessarily lead to the durable establishment of new practices. Moreover, knowing the benefits of a solution or adhering to an objective does not always suffice to modify practices. Behaviors depend on habits, social norms, material constraints, and the context in which they are embedded. A study on the non-adoption of solid shampoo highlights the importance of habits and the product's image, with responses also pointing to functional barriers such as storing the bar under the shower, the sensation left on the hair, and performance differences between products. Consumers may wish to reduce packaging and water transportation while continuing to seek the sensory qualities they have become accustomed to. The challenge then becomes whether eco-design should replicate the expectations shaped by decades of innovation or if evolving expectations could be part of the solution.