butter wouldn't melt in (someone's) mouth

if butter wouldn't melt in someone's mouth, they look as if they would never do anything wrong although you think they would.

She looks as though butter wouldn't melt in her mouth but I've seen her fighting with the younger kids.

• cut through something
• cut through something like a hot knife through butter
• cut through something like a knife through butter
• cut to the chase
• cut to the quick
• cut two ways
• cut up rough
• cut your coat according to cloth
• cut your coat according to your cloth
• cut your losses
• cut your own throat
• cut your teeth
• cut your teeth in
• cut your teeth in something
• damned if do and damned if don't
• damned if do and damned if you don't
• damned if you do and damned if don't
• damned if you do and damned if you don't
• damn it
• damn someone with faint praise
• damn something with faint praise
• damn with faint praise
• damp squib
• dance to someone's tune
• dance to tune