the (easy and delicious) celery soup

1 large celery root (or two or three little ones) (celeri rave)
salt/white pepper (preferably)
1 pot of yogurt
1/2 lemon

peel the celery root, cut it up in little squares.
place in a pot, cover with water (or a touch more)
bring to boil, add salt and pepper. let simmer for 15 minutes.
add yogurt and sqeeze half a lemon.
blend thoroughly.
add a touch of cream or/more water if needed. serve.