Give your brain some exercise — try and solve this brain teaser.

A short riddle making the rounds online sounds impossible at first:

“Two girls have the same parents. They were born at the same hour of the same day of the same month. But they aren’t twins. How is this possible?”

At a glance, most readers jump straight to the obvious conclusion: twins. Same parents, same birth time, same date—what else could it be?

But the riddle hides a simple detail in plain sight. It never says only two children were born.

If the two girls were part of a multiple birth with more than two babies, then they can share the exact same birth hour and date, have the same parents, and still not be twins.

The correct answer: They’re part of a set of triplets (or quadruplets), so they aren’t twins.

In other words, the two girls were born in the same delivery along with at least one more sibling, which makes them triplets (or more)—not twins.