is/was ?

[Biro]

Can one say " "Some of the milk we had at breakfast is sour." Or is only the past tense "was" possible ?

[Luschen]

I think they can both be correct, but have different meanings.

Some of the milk we had at breakfast is sour. - The milk was not sour at breakfast, but it is sour now. I don't know why you would keep milk from breakfast though. Perhaps it is the rest of the milk left in the jug or in the glasses. My kids always leave half-finished glasses of milk around the house; I usually smell it first then add it to my coffee.

Some of the mile we had at breakfast was sour. - The milk was sour when we tried to drink it at breakfast.