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“Every drop has a path”: a journey through the water cycle explained to children at Técnico

February 9thupdated on March 18th at 12:08

The talk “Explain it like I’m 5” highlighted the role of civil engineering in the journey of water to cities.

Before there are pipes, taps, or treatment plants, there is a drop of water. Suspended in a cloud, it falls, slides down a leaf, seeps into the ground, or flows into a river. This journey of water was the focus of Filipa Ferreira, a professor at Instituto Superior Técnico, Universidade de Lisboa, and a researcher at the Civil Engineering and Innovation for Sustainability (CERIS), who led the 5th session of “Explain it like I’m 5”, held on 6 February 2026, at Técnico – Alameda Campus (Congress Centre).

“Did you know that there are cities where every drop of water counts… and others where every drop is wasted?”. This question launched the theme of the session and the first key idea: water covers most of the planet’s surface and makes up a large part of the human body, but the fraction of fresh water available for human consumption is limited. “Managing water means ensuring that this limited resource reaches people in good quality and returns to nature with the least possible impact”, explained the researcher.

The talk began with the natural water cycle – evaporation, cloud formation, condensation and rain. Filipa Ferreira then described the process that occurs when a drop of water enters a city: “We capture water in nature, treat it to make it safe, store it and then distribute it through pipes to homes”. After it is used, the water carries waste and pollutants. “We cannot return it to the environment like that. That is why there are wastewater treatment plants [a topic that was also covered in the December talk ’What happens to toilet water?], where the water is treated before it is returned to rivers or the sea, so that it can be reused in various contexts”.

The contrast between different urban realities was highlighted. There are cities where every drop counts… networks are repaired to prevent leaks, rainwater is collected, reuse is planned and, on a larger scale, dams and reservoirs are built. In other cities, every drop is lost… access to drinking water is not guaranteed, infrastructure is insufficient or degraded, and people travel long distances to obtain water. “Clean water is a privilege. We must know how to value and preserve it”, the researcher emphasised.

“Civil engineers don’t just work on houses, bridges or roads. They also design water networks, drainage systems and treatment plants”, said Filipa Ferreira, describing the work that connects design, construction and operation of infrastructure. The professor also explained that CERIS researchers develop “studies to improve water and sewage treatment technologies, increase network efficiency, adapt systems to climate change and study new and safe ways of reusing water”.

“What if there is a drought?” someone in the audience asked. The question brought the conversation back to water management and the importance of storing water, creating reserves, protecting aquifers and, above all, saving water. Examples of large urban drainage infrastructures in Lisbon illustrated the scale at which civil engineering can operate, including large-scale underground projects. “The ideas are all a little crazy”, joked Filipa Ferreira when confronted with the idea of the “mad scientist”. “Many solutions start with ideas that seem unlikely. Research involves testing solutions to problems that don’t yet have an answer”, she concluded.

“taking shorter showers”, “turning off the tap while brushing your teeth”, “reusing rainwater for watering” were some of the answers from the audience to the question “what can we do to save water?”, associating everyday gestures with the urban water cycle. “Every drop has a path. What we do at home is also part of that path”, said Filipa Ferreira.

By the end of the session, the drop of water had already completed a journey through clouds, rivers, treatment plants, underground pipes, and notebooks. It was drawn on sheets of paper, highlighted in notes, and transformed into “superpowers” printed on stickers, ready to start another journey elsewhere.

The series of talks ‘Explain it to me as if I were 5 years old’, aimed at primary school children, aims to bring scientific research closer to society by promoting direct contact between scientists and children.

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