WASHINGTON (AP) — Using videos that claim to teach toddlers, or flash cards for tots, may not be the best idea. Simply talking to babies is key to building crucial language and vocabulary skills — but sooner is better, and long sentences are good.

So says research that aims to explain, and help solve, the troubling “word gap”: Children from more affluent, professional families hear millions more words before they start school than poor kids, leaving the lower-income students at an academic disadvantage that’s difficult to overcome.