By using the pen tool, we normally need to clear the strokes in the beginning (for example, when "start" pressed). However I just realized there is a small bug in Scratch. The "clear" function inside pen block does not have universal effect. If there are other sprites who have interactions with pen (touching color for example), you need to add function "clear" to the scripts of all the sprites.
I realized this problem by programming the following project:
Instruction: Left mouse click on a position to start building net. The aim is to catch as many butterflies as possible.
There is a short demo of the net building and catch in the beginning. The game starts when it says "Ready? GO!".
Link to the project: http://scratch.mit.edu/projects/14902576/
