matching bruises

This morning the boys were running around the house and all the sudden started screaming.  They came into our bedroom holding their heads.  They got in trouble for screaming.  After they were done cleaning their room, they went back out to play.  I got dressed for church and went out to the front room to tell the kids to get dressed.  Alex informed me that he wasn't going to church.  "Why not?"  I asked.  "No one wants to see my bump."  he says.  What bump?  I then found out that the boys had run into each other really hard and each had a bump on their heads.  Good thing we have two ice packs for such an occassion.  Too bad that they wouldn't keep them on.  Alex's bump is bigger than John's.  What I wonder is how they lined up their heads so well.  John is taller than Alex.