Children with siblings = better on false-belief tests (b/c more opportunities for pretend play, more interactions involving deception or trickery) Concrete-operational period: children are acquiring cognitive operations and thinking more logically about real objects and experiences. Conversation: a 7 year old can decentre by focusing simultaneously on both the height and width of the, displays reversibility- ability to mentally undo the pouring process and imagine the liquid, relational logic in its original container. Mental seriation: allows one to mentally order a set of stimuli along a quantifiable dimension (ie. height, weight) Transitivity: ability to recognize relations among elements in a serial order (ie. if. A is more than b and b is more than c, then a is more than c: the sequencing of concrete operations. Horizontal decalage: a child"s uneven cognitive performance; an inability to solve certain problems even though one can solve similar problems requiring the same mental operations.