How many miles do you think you can run? Answering questions like that can enhance your workout. Dr. Jane McGonigal is a performance studies expert – and she says simply making a prediction in your head triggers production of the feel-good chemical dopamine… which can help you work harder without feeling like you’re exerting yourself more. Because the increased dopamine gives you a bigger sense of determination – and distracts your mind from focusing on the effort you’re making. So just before you start your run, guesstimate how long it’ll take to do your first mile. You’ll end up pushing yourself to make that time, or beat it.