added an if statement to catch when we click the last cup to toggle it

drink-water/script.js

@@ -10,6 +10,7 @@ smallCups.forEach((cup, idx) => {
 })
 
 function highlightCups(idx) {
+    if (idx===7 && smallCups[idx].classList.contains("full")) idx--;
     if(smallCups[idx].classList.contains('full') && !smallCups[idx].nextElementSibling.classList.contains('full')) {
         idx--
     }