HMMmmm .. is it gravity ?? .. is it glue ?? .. maybe tie-wraps ??... OK, we give up ! what holds it in place ??
Now that I got the "smart-arse" answer over with, if you want us to help, we need a bit more information (see example) about your machine, so we can go looking for the information we have available to us, then relay that to you !
Fuel tank on a 10323 is just secured by the engine shroud and an aluminum 'shelf' built into the engine. No screws hold the tank into place it's just sort of boxed in by the shroud, I think there are some foam blocks to hold it also.