One of the other complexities of English phonetics has to do with whether [θ] and [ð] are truly interdental (produced with the tongue tip protruding between the front teeth). For many people they are, but for a sizable number of perfectly good speakers of English, [θ] and [ð] are dental (produced with the tongue tip touching the back of the upper front teeth). Ask your peers (number of three) to help you look at this variation.
1. Make a list of words that contain [θ] and [ð] (preferably 10 each).
2. Watch while your friend reads the list. Or you can record a video of his/her mouth movements for thorough observation. Note briefly your peers’ language backgrounds.
3. If you can see the tongue protruding, your friend is making [θ] and [ð] interdental. If not, then they are probably dental. Do three of your peers say [θ] and [ð] the same way?