My career started when I was a child and I built my first sandcastle on the beach in Genoa, where I grew up. Making things has always been a pleasure for me – happy hands, happy mind – and making sandcastles was my training in fantasy. Now, as an architect constructing buildings like the Shard, I have to think about the final result, but as a child making castles of sand I didn’t, they were ephemeral.

I have four children; the oldest is 50 and the youngest 16, so I have been making sandcastles for a long time. There is no age limit – you can enjoy making a sandcastle however old you are, although it helps to think like a child. Here’s how to do it:

Italian Architect Renzo Piano in New York. Photograph: Richard Drew/AP

1 Be clear about the fact that building a sandcastle is a totally useless operation. Don’t expect too much; it’s going to disappear, mainly because there’s no point making the castle too far away from the sea. A sandcastle’s relationship to water is more important than its appearance. Study the waves, then decide where to position your castle – too low on the shoreline and the sea will immediately destroy it, too high and you have no waves to flirt with. It sounds complicated but it’s simple and instinctive.

2 Start to dig a ditch where the waves have made the sand wet. Use your hands. Build the sand up to create the mass of the castle, which is really a little mountain with an incline of, ideally, 45°. You don’t need the ditch to be more than 30cm deep and 45cm wide, and the castle should be about 60cm tall.

3 Make an entrance in the ditch for the sea to enter. The magic moment is when the waves come and the ditch becomes a moat. If the castle is in a good position, you can watch the water ebbing and flowing for 10 or 15 minutes. To capture the image in your memory quickly, close your eyes when the water comes in.

4 Then put a little flag or anything else you can find on the sandcastle, just to make it visible to people running on the beach. Go home and don’t look back.

As told to Rosanna Greenstreet