I am drawing a rectangle and a straight line across it (using pencil or pen). Then I select them and use Path->Divide and both shapes disappear while when I draw curved line instead of straight, rectangle divides as I expected. Why is that?

Here is a picture if I did not explain clearly:



Sorry, if it is a stupid question, but I am new to Inkscape and I could not find an answer in manual. Please be understanding.

That's a good question. I would like to know the answer too. In the meantime you'll have to use the Path>Cut Path option and then re-set the fill (as your items will become transparent).

try to give fill and stroke to the both and experiment with closed and opened paths,
if you select both nodes with and close them with "Join two selected end nodes with a path segment.", then do Path->Divide and it will give you effect you want.

@hash

joining both end nodes is a nice little trick. i actually like that better since you don't have to re-select the fills with Cut Path. nice tip.