I'll say the same as everyone else: short but fun. Getting those chain reactions going is really satisfying. I look forward to seeing where this goes from here.

Here's my feedback for the demo:

If I'm being nitpicky then the progression could be a little smoother than it is. Early on the upgrades didn't really seem to make a huge difference in how much money I earned, then all of a sudden you hit the critical point and start getting self sustaining chain reactions making boat loads of money and sweeping up the rest of the upgrades in a few rounds, without much in-between.

There was a few frustrating things. I kept having the screen zoom and moving around when I'm trying to use the heat gun on something, making me miss and waste heat. Making it so the screen doesn't move when you're actively applying heat, or just making the camera entirely player controlled would be nice.

The heat gun speed upgrades ended up making me accidentally double tap and waste heat more than do anything helpful. Having an option to make the heat gun semi-auto or something would be nice. Or maybe make it so it's more like placing bombs where you click instead of needing to hold the mouse over the spot would control better?

Some of the upgrades could use a better description so you'd know what they actually do. e.g. I was confused what the difference between the two heat speed upgrades were.

I noticed that I wasn't hitting cells that I'd think I should based on the reticle, especially later in the game as my pickup radius went up. Does upgrading the pickup radius not change the radius that you hit cells with? If so that makes it hard to aim when the circle gets bigger. Either way I'd probably make the reticle a little smaller so it feels like you're getting away with something if you clip a cell at the edges of your heat radius.