float lineX;
float lineY;
float squareX;
float squareY;
float squareSize;
color Clr;

void setup(){
size(400,200);
noFill();
rectMode(CENTER);
lineX=mouseX;
lineY=height;
squareSize=20;
Clr=color(255,0,0);
}

void draw(){
background(0);
stroke(Clr);
line(mouseX,0,mouseX,lineY);
rect(0,100,squareSize,squareSize);
}