In your mind you plan out a lesson and you have certain expectations of what your students might do or ask and what might happen during your lesson. If you see any potential bad ideas you normally try to point them out and mention that they are not allowed. Some days kids meet your expectations and other days they just bust right out of the box of expected thinking.
We're studying matter and today we were learning about how matter can change states from a solid to a liquid and back to a solid again. So I paired the children up and explained that I would give each pair an ice cube and they could do whatever they wanted to try and get it to melt quickly. My expectations: they will sit it in the sun, rub their hands around it or break it into smaller pieces to get it to melt quickly. Yes, they tried all of those things along with sitting on it and even putting it in their mouth to help the process along...both fairly decent ideas. But then there was the kid who busted out of the box and decided to melt the ice cube by sticking it into their underwear. Oh my! Forgot to cover the rule that says you may not stick the ice cube into your undies....Dear me! What will I do with you little first and second graders?