@book { DasStufenmodellderkognitivenEntwicklungnachJeanPiaget,
    title = "Das Stufenmodell der kognitiven Entwicklung nach Jean Piaget",
    author = "Frank {Becker}",
    year = "2001",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783638974936",
    url = "https://www.hausarbeiten.de/document/99044"
}