You can encourage young children by calling them a "big boy" or "big girl" when they do something impressive for their age like:

• sitting upright without help for the first time
• using the toilet correctly
• cleaning up their toys

You can use this phrase with a child until about age 4. After that, most children feel like they're already a "big boy" or "big girl", so they don't want to be called that.

Calling an adult a "big boy" or "big girl" has a completely different meaning. It means that they're fat.