X Marks the Spot

Aarrgghh....last week I was cleaning up before care group and Easton and Jed were playing hide and seek (or so I thought). Easton came to me and asked me what X stands for, does it mean x marks the spot? Sure I say and go about my work. A few minutes later I notice an x on the coffee table. I swatted his hand and told him not to color on the table...a week later I am still finding x's everywhere.