It started in a school WhatsApp group.
Our kids' school in Barcelona has an intercambio — a WhatsApp chat where families pass on outgrown clothes and toys. It works — just not always well. Things get buried. Items go to whoever happens to be online at the right second. There's no record of what's available or who claimed what.
But the underlying idea — families helping families, things kept in use, nothing thrown away that's still good — that idea is right.
Outgrown is that same idea, but built to work better. Bigger than one school, smaller than the whole internet. Local enough to meet at the park, large enough to actually find what your kid needs next.