GIMP, a reminder

I use some GIMP functions sometimes:

Free Select Tool: Click the small circle in the first point to close out (or use 'enter' key, not double-click as I assumed), use different 'mode' to add, subtract, etc.
Path tool: some are similar with free select tool when you just mouse-click, use 'enter' to close out. When using mouse-drag, it creates curve.
In Fuzz tool, you can also use 'mode' to 'add' some region to get a big region (sometimes the color inside the object is different because of the threshold)
Select by Color Tool: similar with Fuzz tool, just select multiply objects (not continuous as Fuzz tool)
Scissor Tool: choose region based on contrast. After you clicked two points, you can click the middle of line to refine it. Hit 'enter' to get the selection. Tool clean up the selection, use 'Free Selection Tool' to subtract the selection.
Mask: to refine the selection. With brush tool (default color), black (foreground) is applied in mask, and white is applied the others.