Inter-textual exploration

Using a Tanach, Look up the following references. Is there any difference between the meaning of  ( אות ) in these passages? What do you think the function of the ( אות ) is in each case?  (to show? tell? remind? something else?)

• Bereshit 4:15  (Cain and Abel)

• Shemot 31:13 (Shabbat)

• Shemot  31: 17 (Shabbat)

• Shemot  8:18-19  (Plagues)

• Shemot 12:13 (Blood on the doorposts)