Trick question! There isn’t one. It’s not what’s happening in your mouth that makes these sounds different. It’s the “voice onset time”—the time between when you start moving your tongue and when you start vibrating your vocal cords. If that time is greater than roughly 40 milliseconds, English-speakers will hear ta. If it’s less than 40 milliseconds, they’ll hear da.

What’s amazing is that you never hear anything other than ta or da. If two speakers fall on the same side of the 40-millisecond dividing line, it doesn’t matter if their voice onset times differ dramatically. One person’s time might be 80 milliseconds, and the other’s might be only 50 milliseconds, but in both cases you’ll hear ta. If their times fall on opposite sides of the divide, however, a difference of just 10 milliseconds can be transformative. If one person’s voice onset time is 45 milliseconds, you’ll hear ta. If the other person’s time is 35 milliseconds, you’ll hear da. Strange but true.

Read this brilliant HBR article to find out more!