Cooper Hewitt, Smithsonian Design Museum

Listen for a moment. The traffic on Fifth Avenue fades as you pass through the iron gates. Gravel and paving give way to softer ground, and the air smells faintly of cut grass and old stone. Ahead of you sits a broad red-brick house with pale limestone trim. It looks less like a museum and more like a family home. That is no accident, because for many years this was exactly that.

You are standing at Cooper Hewitt, Smithsonian Design Museum. It is the only museum in the United States devoted entirely to historic and contemporary design. Here, a teaspoon matters as much as a painting. A chair, a poster, a scrap of wallpaper, a typeface... all of them are treated as serious records of how people live. The building that holds them carries its own story, and it starts with one of the richest men in American history.

This is the Andrew Carnegie Mansion. Carnegie was a Scottish-born industrialist who made an enormous fortune in steel. He then spent much of his later life giving that fortune away, funding libraries, concert halls, and universities. He built this house at the very start of the twentieth century, and it was finished in 1902. The style is Georgian Revival, a look borrowed from English houses of the eighteenth century, with symmetry, red brick, and calm proportions.

Here is the first surprise. When Carnegie chose this site, it was far north of the fashionable part of Manhattan. Most wealthy families clustered further downtown. Carnegie wanted space, light, and a large garden — rare things on this crowded island. He bought land others ignored. Today, the whole neighbourhood around you is called Carnegie Hill, named after the man who moved here first.

The mansion looks traditional, but inside it was a modern machine. It was built on a steel frame, which suits a steel magnate. It had central heating and an early form of air cooling. It also had one of the first private passenger elevators in a New York home, made by the Otis company. So, before a single design object arrived, the house itself was already an example of new technology hidden behind old-fashioned walls.

Carnegie lived here until his death in 1919. His widow, Louise, stayed on for many years after that. Later, the house served other institutions, including a school of social work. Then, in the 1970s, a new chapter began. The Smithsonian Institution, the national museum group based in Washington, moved its design collection into these rooms. The museum opened here in 1976, and a private home became a public one.

The collection is older than the museum's address. It began in 1897 with two sisters, Sarah and Eleanor Hewitt. They were granddaughters of Peter Cooper, an inventor and businessman who founded the Cooper Union, a free college in lower Manhattan. The sisters had visited the decorative arts museum in Paris and wanted something similar for New York. They wanted a place where students and craftspeople could study real objects up close.

That practical spirit is the second surprise. The Hewitt sisters did not collect only treasures. They gathered samples, swatches, sketches, and fragments, things a working designer might learn from. They believed a good textile pattern or a well-made hinge could teach as much as any masterpiece. Today the collection holds over two hundred thousand objects, covering many centuries and many cultures. Only a small part of it is on display at any one time.

As you walk through, notice how the house and the exhibitions talk to each other. Modern objects sit under carved ceilings and beside heavy wooden doors. Displays change often, so a room you saw last year may look completely different now. One space worth finding is the former library, known as the Teak Room. Its walls are lined with dark carved teak, made in India for the American designer Lockwood de Forest. The carving is dense, patient work, and it rewards a slow look.

The museum went through a long renovation and reopened in 2014. That project opened up more of the house for galleries and added interactive features. One of these is a digital room where visitors can explore wallpaper designs from the collection, projected across the walls around them. You can even draw your own pattern and watch it fill the room. It is playful, but it also makes a serious point. Wallpaper is design you live inside.

In recent years, the museum has also experimented with digital tools that let visitors save objects they like and look at them again later. The idea follows the Hewitt sisters' original mission. A museum of design should not just be looked at. It should be used, studied, and carried home in some form. Ask at the front desk what interactive features are running during your visit, since they change over time.

Before you leave, step out into the garden. This is the green space Carnegie wanted when he came this far north. Lawns, trees, and paths sit behind the iron fence, and the city noise drops to a low hum. In warmer months the garden is a calm place to sit and think about what you have just seen. Many visitors miss it entirely, heading straight back to the avenue.

So take a last look around. The red brick, the limestone trim, the tall windows catching the afternoon light. A steel fortune built this house, and two determined sisters filled it with ordinary, beautiful things. As you walk back toward Fifth Avenue, notice the design all around you — the shape of a bench, the lettering on a sign, the curve of a lamp post. Once you start seeing it, you will not stop.