In an elementary setting, I believe keeping the objectives confined to the here and now is essential. I am finding that having a subject's, or an integrated lesson's, objective(s) posted on the board, along with a list of planned activities (optional), is helpful for both students AND the teacher. As long as students can easily see the board, teachers can quickly gesture in reference to the objective/activity space in response to a student's inquisitive probing. As a teacher, you have your own reference point to help keep track of when and where you're going in a day. Granted, you may have been dreaming and practically surviving off of the thoughts that run around the lessons you plan to teach - but there are oh so many hats to wear and distractions that potentially come up in a day. Safe to say, visible and clear objectives/activities is a useful anchor point for all.
"Ohhh, so we're going to learn about how to use nonfiction text features to help us answer our questions?" As the teacher, you enthusiastically nod your head. What follows next is a well thought-out and organized approach to teaching this concept, with lots of modeling and gradual release of responsibility (something else that is easier said then done effectively!) - a post for another day. :) So don't hesitate or feel that you need to hide plans or stifle your kids' great questions - tell your kids what they are going to learn next!