Interesting. I've never thought of washing rice before cooking it and in my area (north Italy) rice is quite a traditional food. UPDATE: I've done some research and I confirm that rice isn't washed anymore in Italy (it was customary to wash it, bu now packaged rice is generally considered safe for consumption without the need of washing it). Besides, in North Italy the traditional way of cooking is "risotto" (http://en.wikipedia.org/wiki/Risotto) that needs all the starch of the rice to create its typical creaminess. Rice used for other dishes is (i.e. rice salds) is often washed, in the colander, after cooking, both for cooling it and to remove starch (so that the grains remain detached) .
sgtluax is correct, but just to clarify:
"Mein" can be either masculine nominative (singular) or neuter nominative (singular). It could also be neuter accusative (singular).
- Mein Reis ist weiß. (My rice is white; [der] Reis is masculine and nominative [singular])
- Mein Auto ist weiß. (My car is white; [das] Auto is neuter and nominative [singular])
- Sie wäscht mein Auto. (She washes my car; [das] Auto is neuter and accusative [singular])
"Meinen" can be masculine accusative (singular), as in this exercise.
- Sie wäscht meinen Reis. (She washes my rice; [den] Reis is masculine and accusative [singular])
"Meine" can be feminine nominative (singular), feminine accusative (singular), nominative plural, or accusative plural.
- Meine Katze ist weiß. (My cat is white; [die] Katze is feminine and nominative [singular])
- Meine Katzen sind weiß. (My cats are white; [die] Katzen is feminine and nominative [plural])
- Meine Hunde sind weiß. (My dogs are white; [die] Hunde is masculine and nominative [plural])
- Meine Autos sind weiß. (My cars are white; [die] Autos is neuter and nominative [plural])
- Sie wäscht meine Katzen. (She washes my cats; [die] Katzen is feminine and accusative [plural])
- Sie wäscht meine Hunde. (She washes my dogs; [die] Hunde is masculine and accusative [plural])
- Sie wäscht meine Autos. (She washes my cars; [die] Autos is neuter and accusative [plural])
*Note that there may be additional possibilities/exceptions that I am forgetting, but this is the general pattern. Well, as best I can describe it.
Hope this helps a little.
If it's a direct object or follows a preposition that requires an accusative, then you need the accusative case. Have a look at this and see if it explains it better than I can:
To cook rice, you have to wash it by rinsing it several times before. It is to clean the rice from dirt, dust, insects, and chemicals (sometimes there are residues of them). They're very common to be found in rice as they are usually stored in sacks in warehouses. This sentence means literally washing rice. In a similar fashion you would wash fruits, vegetables, meat and alike.
My mother would wash the rice before she cooks them for dinner. Therefore, she is washing our (family) rice.
The verb would be different: Sie waschen .... See the full conjugation here, and notice the pattern when comparing with other verbs.
A shortcut for remembering this is that regular verbs end in -t for er/sie/es ("he/she/it") and end in -en for wir/sie/Sie (we/they/you (formal)). Even though waschen isn't regular, that trick still works in this example.