Here’s how it works: When you learn something new, fibers called dendrites branch out of neurons or nerve cells in your brain like twigs on a tree, forming dense networks. Electrical signals transmit information from one neuron to another across a small gap at the contact point called a synapse.

“When you’re building those connections, you’re actually changing the shape and structure of neurons,” Amen says. “You’re pumping them up and giving them more contact points to learn new things.” Although some synapses are weak, learning makes them stronger.

If you’re depending on crossword puzzles to keep your mind strong, you may want to rethink the strategy, Amen says, since doing the same thing over and over again isn’t challenging enough to work that magic. “It’s like going to the gym and just doing bicep curls,” he says. “Your brain has many different parts, so you want to engage it in different ways. The act of mixing it up makes the brain more agile.” —Beth Howard