You find it stuffed at the back of the wardrobe, musty, a little worn-out. On the front perhaps there is a cartoon camel or a number that for four weeks you were inexplicably wedded to. On the back, the names of people you've long forgotten, with jokey nicknames you no longer understand.
And suddenly, you're 16 again.
Perhaps it's not the T-shirt that brings back that summer. Perhaps it's a song, sung around a campfire by a group of sun-burned teenagers. Or the smell of burnt schnitzel and hiking boots after a month of wear, or faded snapshots of people you swore to stay friends with forever.
This week, the annual exodus of British teenagers to Israel begins once again. As they have for decades, parents will wave their offspring goodbye, fretting over the number of Primark T-Shirts and pairs of socks packed. Is one set of water shoes enough? Have you got plenty of spending money? Don't forget to call.
Forget a barmitzvah or a bris; Israel tour is the real rite of passage. The hikes in the Ramon Crater, the first glimpse as a group of the Western Wall or the Shabbats under the stars in the Negev stay there for a long time, perhaps forever.
Sixteen year olds can be cruel, romances fade fast
That month - that blissful, glorious summer after the slog of GCSEs, the one where you first develop a love or hate attitude towards humus, learn the words to Baruch Hageva and buy lurid 'Eilat trousers' at an overpriced market stall - that's what it means to be a Jewish teenager.
There's education in Zionism, biblical and modern Israeli history, Jewish life and the ideology of your youth movement. Having been on tour twice - once as a participant, once as a leader - I can't say all of what is taught sticks.
Still, the afternoon at Tel Aviv's Independence Hall, the visit to Yad Vashem or the schlep up Masada at dawn to relive the Roman siege, are more than just 'the bits between free time'.
Tour isn't necessarily the 'best month ever' or even as 'amazing' as the subsequent Rosh Hashanah chatter would suggest. As a leader, I warned that with the good times, there would be tears. Sixteen-year-olds can be cruel, summer romances can fade as fast as they spark and the lack of sleep for 28 solid days will inevitably catch up.
I've lost count of conversations I've had with people who felt that not everything lived up to their expectations, though they'd have died rather than admit it at the time.
But for all that, it's still one of the most important things offered by British Jewry. It's not because we have to instil in teenagers an unblinkered love for Israel, or a love for all things Jewish, but because it opens the door.
Even those who holiday in Netanya every summer come away with a new perspective, a new memory. They return home with souvenirs, dirty laundry and best friends, but also with endless opinions on Israeli politics or lifestyle, a stronger awareness of their heritage and more questions about who they are than they left with.
Look around our university campuses, at the projects being set up and at the people making a difference to modern Jewish life. Many, if not all, will trace their passion back to four weeks of youth hostels, hikes and night stays in the Bedouin tents.
Yet for all the hundreds who are writing luggage labels and sitting on their suitcases this weekend, still more will be missing out. They'll be other chances - Birthright is perhaps the best example - but too many will never take them. As we say at Pesach and Yom Kippur, next year in Jerusalem - for every British Jewish teenager.
Tour isn't just a summer holiday and it shouldn't be taken for granted. Because the people who will lead our community in future will more than likely have started their journey at an ElAl check-in desk wearing a brightly coloured T-shirt.
