Generally speaking, we are born with all of the gear we’re ever going to have: Heart and lungs are fully formed; the maximum number of fingers and toes are in place; infant ovaries contain the total number of eggs that the female human will ever release, regardless of how many hundreds of years she might live. The brain, on the other hand, is a little trickier. Although we are born with the total number of neurons we’ll ever have (more or less), these are largely unwired at birth. It is the connections between neurons (called synapses) that are the gross physiological feature responsible for juggling, remembering phone numbers, and signing a bill into law. During the first three years of life, the brain is furiously wiring up these neurons; a toddler’s brain is roughly twice as active as an adult’s and twice as dense with synapses—toddlers have about one trillion. The brain vastly overwires in the first three years and then spends childhood and early adolescence thinning out the unneeded connections on a use-it-or-lose-it basis (a process that developmental researchers call pruning). From toddlerhood forward, a child is performing relentless, round-the-clock experiments to figure out which synaptic connections are important in her environment (What language is spoken here? How important is foraging? Are stairs an issue?), reinforce those connections, and lose the ones that aren’t applicable. (This is why Hispanic toddlers can effortlessly trill their rrs and Anglo-Midwestern college students can’t.) More than genetics, our environments form our brains, and they do it years before we ever pick an extracurricular activity or say “I wanna learn to play piano.”

In a strict evolutionary biology sense, the job of any toy is to exercise as many of these synapses as possible, so they don’t get lost. The following toys and projects are fun and encourage open-ended creative play, exploration, creative expression, curiosity, strategic planning, and impishness.