The Conditional Challenge Quiz - Feedback?

First up, questionsLeft wasn't declared first as a variable using the var keyword, so it will be a global variable. You should include it at the top:

var questionsLeft;

OR, instead of repeating the line:

questionsLeft=' ['+ questions+' questions left]';

...you could make a function called questionsLeft (instead of the variable above) and have it return that value:

function questionsLeft() {
  return ' [' + questions + ' questions left]';
}

Then you would call the function from within each prompt:

var q1=prompt("What is the lightest element in the Periodic Table?" + questionsLeft() );

Alternatively, you could create a function to use the prompt and display the questions left in one go:

function askQuestion(question) {
    return prompt(question + ' [' + questions + ' questions left]');
}

var q1 = askQuestion("What is the lightest element in the Periodic Table?");

You can also chain toLowerCase() directly to the prompt() function call:

var q1=prompt("What is the lightest element in the Periodic Table?" + questionsLeft ).toLowerCase();

Also, it won't change much, but an alternative syntax to adding or subtracting 1 that you could use is:

questions--;  // instead of questions -= 1;
count++;      // instead of count += 1;