Als de kat van huis is, dansen de muizen — “When the cat is away, the mice dance.” This idiom is a Dutch equivalent of the English “When the cat’s away, the mice will play” — when the authority figure is absent, the subordinates do as they please. The Dutch phrasing uses dansen (dance) rather than “play,” giving it a slightly more joyful and less negative connotation.
Usage contexts: a teacher leaves the classroom (Als de leraar weg is, dansen de muizen). The boss is on holiday and the office relaxes (De baas is op vakantie — als de kat van huis is, dansen de muizen). Parents leave for the weekend (Nu de ouders weg zijn, dansen de muizen). The idiom is universally understood across generations and regions in the Netherlands and Belgium.
Animal idioms are a rich category in Dutch: Het regent pijpestelen (It is raining pipe stems = it is raining heavily), Een paardenmiddel (a horse remedy = a drastic cure), Vlinders in je buik hebben (to have butterflies in your stomach — same as English), De hond in de pot vinden (to find the dog in the pot = to arrive after all food is gone), Katten en honden regenen is not Dutch — use pijpestelen regenen instead.