Structure
It is very common in English to put two nouns together without a preposition.

• tennis shoes    
• a sheepdog    
• the car door    
• orange juice

The first noun is like an adjective in some ways.

Compare:

• a race-horse ( = a sort of horse) a horse-race ( = a sort of race)
• a flower garden  ( = a sort of garden) a garden flower  ( = a sort of flower)
• milk chocolate    ( = something to eat) chocolate milk    ( = something to drink)