A simple line drawing of a duck--or is it a rabbit (?)--offers loads of fun for children and adults in the beloved picture book, Duck! Rabbit! by Amy Krouse Rosenthal and Tom Lichtenheld. Two narrators, constantly at odds with one another, propel the story forward by arguing about what they see on the pages of the story. It's a delightful twist on the classic optical illusion that provides kids with the opportunity to defend what they see, but also to try and see another's perspective.
For a time-honored picture book that encourages children to use their imaginations in a similar way, try It Looked Like Spilt Milk by Charles G. Shaw. A predecessor to Duck! Rabbit!, this story cannot help but remind readers of a Roschach test, albeit without any psychological implications. Kids will enjoy calling out what they see, especially if it's extra wacky. Stories and activities like this help children to become more familiar with shapes, which is vital in learning the written alphabet and which will come in handy for future math classes (particularly geometry). And, remember that you don't need a book to do this, you can always just step outside and look up at the clouds!