Available in three different levels, they are fun for all ages. Our family worked our way through the entire series this summer, then moved onto Spelling DooRiddles once school started. The lower levels are fairly easy — our older kids would whisper the answers to their younger siblings whenever the little ones got stumped — but the upper levels can be surprisingly challenging. I would read about a dozen riddles aloud to my kids just before our afternoon storytime. They’d race to see who could solve it first, then I’d discuss and/or explain the solution before moving onto the next riddle.
I have seen marked improvement in verbal reasoning in all the kids who participated, from age four to fourteen. Our family has been using materials from the Critical Thinking Company for over twenty years now, and highly recommend them. We’d encourage you to check out all they have to offer, including a slew of free printable activities for your students. (And if you’ll use this link to get there, they’ll give you free shipping.)