Toddler laughs when she's told off

We have a delightful and happy 21 month old. She is full of life and so energetic. She's sweet and loving but sometimes when we need to tell her off for things such as running off, not listening or throwing her toys she just laughs in our face. She will say sorry mum with a big grin and if I'm stern and say I'm not happy and tell her what she's done she just laughs! I'm a teacher and behaviour managent is one of my strengths. I can get the toughest of kids to stop with a look and stern word but my 21 month old......no chance!!!! Any advice? Is this a normal