Eli had considered how to approach this situation at some length and had decided that a veiled sort of honesty was the best approach. "I would like to put a memory in your head." He popped the top off the vial and guided his father's memory into the pensieve for Roger to review before agreeing or disagreeing.
What he wouldn't see was the memory that would be attached to it. The one he had carefully created to test his other theory. Sure, it would be messy work testing two theories at once, but he also believed that the memory would incorporate more fully if it was woven with another foreign memory. Especially if that was what those were.