It's simple game: it starts by counting from 1 and when the counting reaches 7 or number that contains 7(ex: 17 ) or one of seven's multiplies(ex: 14, 21,28 ...) the person says BOOM! instead of that number.
ex: person 1 : 1
person 2 : 2
person 3 : 3
person 4 : 4
person 5 : 5
person 6 : 6
person 7 : BOOM!
person 8 : 8 and so on.
If one person forgets to say BOOM! the counting restarts from 1.
I start: 1