Not a stupid question at all.
35 Remington, second from right, .357 mag, leftmost:
35 Remington:
Rim diameter: .460 in
Base diameter: .458 in
bottlenecked rimless case
.357 Magnum:
Rim diameter: .440 in
Base diameter: .379 in
straightwalled rimmed case
A cartridge case is not designed to contain the pressures of ignition. It is made from thin and relatively soft metal (brass). The strong and thick-walled steel chamber contains the pressure, the cartridge case just acts as a gasket for the most part. Looking the above numbers you can clearly see that a .35 Remington case could never fit in a .357 chamber, and vice versa would be dangerous since the .357 Magnum base diameter is extremely undersized for a .35 Remington chamber. Often it is also the case that pistol calibers run higher pressures than rifles, as well, which is a further danger. For instance, the .357 Magnum's SAAMI maximum pressure is 35,000psi, while the .35 Remington's SAAMI maximum pressure is 33,500 psi. Now considering that the .35 Rem chamber is too big to contain that little 357 case, so that it's basically guaranteed to let go, and the pistol cartridge trying to exert that much pressure...you'd have serious problems.
This is all assuming that you'd be able to even get ignition on the cartridge. The .35 Rem is a rimless case, meaning it headspaces on the shoulder, while the 357 is a rimmed cartridge, meaning it headspaces on the rim.
In short just keep ammo that is not an exact 100% match out of your chamber unless you know for a fact that you are dealing with an exception such as shorter/weaker versions of a given revolver chambering such as .38 special/.357 magnum, .44 special/.44magnum, .45 Colt/.454 Casull, etc.
When in doubt only stick to ammo that specifically matches the chamber of your firearm, or ask around to people in the know just as you have, and consult SAAMI specifications. SAAMI has a number of publications on their website that help here, such as their "Unsafe Firearm-Ammunition Combinations" datasheet:
http://www.saami.org/specifications...ublications/download/SAAMI_ITEM_211-Unsafe_Arms_and_Ammunition_Combinations.pdf
Some of these are counter-intuitive, so it's good to always check. For instance, it is generally not safe to fire 5.56X45 NATO military ammo in a rifle chambered for .223 Rem, but .223 Rem in a rifle chambered for 5.56X45 NATO generally is safe, etc.