I'm a little confused by your question. Where I live Grade 7 is in middle school. An explanation for a 13 year old would be very different than one for a primary level student. Indeed, I'd struggle myself trying to explain this to, say, a 2nd grader! The 13 year old however could understand that the water molecule, H2O, is a unique and unusual structure in nature and is bi-polar, with a + and - charge on different ends of the structure. Turpentine does not have such a structure. Because of its structure water is called the universal solvent and can dissolve many solids that petroleum-based liquids like turpentine cannot.