public void RandomChoose() //Chossing a random cube
{
System.Random r1 = new System.Random();
System.Random r2 = new System.Random();
int row = r1.Next(0, grid.GetLength(0));
int column = r2.Next(0, grid.GetLength(1));
var value = grid[row, column];
Debug.Log("choose a random block");
previousRedCude = value.GetComponent <GridItem>();
previousRedCude.ChangeToRed();
StartTime = 0f; //reset the start time to 0
}