@book { DiePhasendergeistigenEntwicklungnachJeanPiaget,
    title = "Die Phasen der geistigen Entwicklung nach Jean Piaget",
    author = "Jessica {Freis}",
    year = "2002",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783638148306",
    url = "https://www.hausarbeiten.de/document/7650"
}