I speak Russian to my mom, but since I've been living in Canada since I was 6 mine is getting a little rusty and I often have A LOT of difficulty conveying anything other than "Are you hungry?" "The weather's awful" and other utterly mundane thoughts.

Trying to explain anything scientific is hell because I only ever went to school in Canada and simply don't have the vocabulary for it... and don't get me started on trying to discuss lucid dreaming