Tell them to "go to the store for a bucket of propwash"
Or "go and get me some striped paint"
Or "this is a left hand thread, so get me a left hand spanner"
And my personal favourite " go and ask fo a long weight" wherby the unsuspecting hairy would ask the toolstoreman for one, and the storeman would wander off for 10 minutes before returning empty handed and say " you just had one"
Thanks Marty for reminding me of these, and I am sure you guys will have many more...

...!