Playing with kids is wonderful informal teaching.
Today we played a lot of make believe games, one of these was playing a petrol pump attendant. Blessed with an amazing creativity my son made petrol pump out of an old toy box wrote some letters over it like P, A, and T with a black marker meaning petrol pump and made petrol pump attendants out of blocks. He then lined up his toy cars as if waiting for their turn to fill petrol.
He filled fuel in the cars with the help of a straw (it came with juice box), and did not forget to ask me for money every time I moved a car up to the fuel pump. He also insisted on checking the air in the tyres, though he said he did not have the equipment to fill the air.
Make believe games are infact the only way I can teach him, without him getting restless and bored. And it is not necessary to have an expensive line up of toys to play make believe games , a lot of knick knacks and blocks are cheaper and best ways for imagination to flow, for example we do not have a toy helicopter but he made one for himself with old CDs lying around in his basket, he attached a string to the CDs and the helicopter is ready to carry anyone or anything.
The only thing that is required is the interest to play with the kid and getting down to his level of understanding but at the same time building on what he is already saying like you can ask him how much is the petrol costing these days? how much money you have to pay him? etc.