[l33t2328]

It is not possible to gain weight by eating your total daily calorie expenditure in pure sugar.

Yes, not all calories are metabolized the same, and sugar may lead to over eating, but at the end of the day, it’s a simple equation. You can’t gain weight without taking in more than you’re putting out.

[nisegami]

You aren't wrong in a technical sense, but this is far too reductionist. The type of calories and the way you consume them can't change the thermodynamics, but they can and do change your hunger, satiation and other bodily sensations that feed back into the thermodynamics by affecting your calories in and calories out. The simplest example is not needing a mid-morning snack because your choice of calories led to you being satiated for longer.