If you are in an area where natural sunlight can shine on your desk, you might be wary of touching it. You might also be wary of placing hotter items onto the glass surface so it doesn’t break under the stress. Glass expands and then contracts under rapid temperature changes and this can cause the glass to break if it is done enough. However, most items (like an overheated computer or a cup of coffee) aren’t hot enough to put a massive amount of stress on the glass. Even direct sunlight isn’t enough as long as you make sure that the glass desktop isn’t exposed to it for 24 hours. Glass desks might get a bit hot or uncomfortable, but unless you have some extreme circumstances they aren’t going to break.