Football crazy hockey mad

In a league of 5 football teams which play in a round robin tournament show that it is possible for all five teams to be league leaders.
Exploring and noticing Working systematically Conjecturing and generalising Visualising and representing Reasoning, convincing and proving
Being curious Being resourceful Being resilient Being collaborative

Problem

Image
Football crazy Hockey mad
In a league of 5 football teams which play in a round robin tournament (i.e. two teams play each other once only with no ties/draws allowed) show that it is possible for all five teams to be league leaders at the end of the tournament.

In a league of 6 hockey teams which play in a round robin tournament (i.e. two teams play each other once only with no ties/draws allowed) show that it is impossible for all six teams to be league leaders.