We investigated the role of children's conceptual understanding and ballgame experience when judging whether a football player is in an offside position, or not. In the offside position, a player takes advantage of being behind the defence line of the opposing team and just waits for the ball to arrive in order to score a goal. We explained the offside rule to 7- and 9-year-old children with a Subbuteo setup. They produced drawings of an offside position until it was correct (drawing to criterion). Thereafter, children judged whether a designated player was in an offside position in a computerized task. Like adults, also children found it easier to judge when a player was in a wrong rather than a right place. Only when including frequency of ballgame practice in the analysis it was revealed that boys were better independently of age as they judged the offside position more systematically. Football, which is called soccer in the US, is a favourite outdoors rule-based game (Christidou et al., 2013). Not only young men (Helsen et al., 1998), also women (Beaudoin, 2006; Lopez, 1997; Scraton et al., 1999) and children love playing football. While for boys, but not girls, becoming a professional footballer is a top choice (Looft, 1971), a ballgame pitch is depicted by all children independently of their sex when asked to draw their schoolyard (Christidouet al., 2013). Moreover, in recent months, the British Football Association (FA) has created multiple initiatives to recruit and develop female football talent (FA, 2016b). Football is often seen as a team activity that fosters social cohesion (Smyth & Anderson, 2001; Watson & Gibson, 1980) even when children are not players, but just football supporters (Spaaij & Anderson, 2010).