To make a lake in Little Alchemy, you will need to combine several elements together.
First, you'll need to combine Water with either Earth or Pressure. The Water element can be found at the beginning of the game, while Earth and Pressure can be unlocked by combining Sandy and Air, respectively.
Once you have Water and either Earth or Pressure, combine both together by dragging one element onto the other to create your lake.
If you would like to replenish your lake, you can add sources of water like Rain, an Ocean, or Sea, which are all available once certain other elements have been unlocked.
You can also add animals or other creatures to your lake for added effect, such as Fish, Frogs or Water Snakes.
Happy experimenting!