Originally published 2019-10-12.
At eight years old, Xóchitl Guadalupe Cruz López from San Cristóbal de las Casas in Chiapas, Mexico, built a solar water heater out of plastic bottles, connectors and rubber hose. Her invention earned her the Reconocimiento ICN a la Mujer from the Institute of Nuclear Sciences (ICN) at the National Autonomous University of Mexico (UNAM), an award normally given to adult women scientists. She was the first child ever to receive it.
Her story spread worldwide, sometimes with the slightly misleading headline that a child had won a "nuclear sciences prize". Here is what she actually built, why it mattered so much in her community, and what the award recognised.
The problem she wanted to solve
The highlands of Chiapas, known as Los Altos, sit well above 2,000 metres. Nights and early mornings are cold, and many families in and around San Cristóbal heat their water over wood fires. Commercial solar or gas water heaters exist, but they are expensive for low-income households.
Xóchitl noticed two consequences. Many people simply bathed in cold water, and she linked that to the respiratory illnesses common in her area. Others cut down trees for firewood, which she saw as harmful to the local forests and, as she put it in interviews, to the climate. As she told reporters, families without money for a heater end up cutting trees for fuel.
Wood smoke in the home is also a health issue in its own right: cooking and heating over open fires fills kitchens with fine particles that irritate the lungs. A heater that runs on sunlight avoids both the smoke and the felling of trees.
How her recycled solar heater works
According to UNAM, the device, sometimes called Warm Bath, is made from easy-to-find recycled items: plastic water bottles, plastic connectors and a length of rubber hose. The materials reportedly cost about US$30. With her father's help, she installed it on the roof of the family home, where it supplied hot water for the household.
Heaters of this kind follow a simple principle. Water flows through dark hose or tubing exposed to the sun, often inside clear bottles that act like small greenhouses and trap heat. As the water warms it becomes lighter and rises towards a storage container, while cooler water sinks to take its place. This natural circulation, called a thermosiphon, needs no pump and no electricity.
Homemade bottle heaters cannot match the output of a commercial system, and nobody has published lab measurements for Xóchitl's design. What they offer is warm water at almost no running cost, built from rubbish that would otherwise go to landfill. A popular family detail from the coverage is that she learned to bathe quickly, so that there was still warm water left for her little brother.
The award, explained
The prize is the Reconocimiento ICN a la Mujer, granted by UNAM's Instituto de Ciencias Nucleares to recognise women's contributions to science. Xóchitl received it in 2018 through PAUTA, a UNAM programme that supports talented children and teenagers in science, as Remezcla and UNAM Global reported.
Despite the institute's name, the award had nothing to do with nuclear technology. It recognised her as a young woman doing science with real-world purpose. Being the first child honoured alongside adult researchers made it newsworthy in Mexico and beyond.
She had been interested in science since she was about four and had already entered several science fairs. Later, TIME magazine included her among a group of young inventors seen as having a better vision of the world, a recognition UNAM highlighted in 2020. At that point she was planning to apply for a patent and to build a larger heater with solar panels, and she had sought support from universities to scale the idea up.
Why stories like hers matter
Women remain under-represented in many science and engineering fields, and girls from rural and Indigenous regions of Mexico face extra barriers, from distance to schools to limited lab resources. Seeing a girl from Chiapas recognised by Mexico's largest university sends a strong message to other children that their ideas count.
Her project also illustrates a principle engineers call appropriate technology: solutions designed around local needs, local materials and local skills. A US$30 heater built from bottles will not change national energy statistics, but in a single home it can mean warm baths, fewer trees cut and less smoke indoors.
Build your own? Some practical tips
Bottle-and-hose solar heaters are a popular school and community project. If you try one:
- Use dark hose or paint the tubing black so it absorbs more sunlight.
- Face the collector towards the equator (south in the northern hemisphere) and tilt it to catch the midday sun.
- Place the storage tank above the collector so the thermosiphon can work.
- Insulate the tank and pipes to keep the water warm into the evening.
- Test the temperature before bathing; on a hot day water can get hotter than you expect.
Teachers often use projects like this to explain heat transfer, insulation and the greenhouse effect with materials students can hold in their hands. Measuring the water temperature every hour across a sunny day and a cloudy day turns a craft project into a real experiment, and it is exactly the kind of testing that would show how well any home-built design performs.
Most of all, Xóchitl's story is a reminder that good engineering starts with paying attention to the people around you.
Sources: Remezcla, 1 March 2018: first child to win the ICN award; UNAM Global, 8 February 2020: TIME highlights the Chiapas girl's invention; Cultura Colectiva (original report); WLRN: Meet Mexico's new famous inventor.
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