Sorry for the extremely basic question but it is something that I can't remember!
What is a reliable way of telling great and blut tits apart? To me, in the garden they look the same and I have trouble telling them apart. embarrassing I know!

Blue Tits have a blue crown and have a much more 'white' looking head (blue crown, white, thin black stripe through eye, then white again.
Great Tits have a black cap which is solid right to the eyes, with under the eyes being white. Also, the black bib on the Great Tit is larger. Overall the impression of the Great Tit is one of darkness, whereas the Blue Tit seems light.
Kingfisher
I usually look for the blue crown for Blue tites and the black for great tits as Kingfisher says as I find this the easiest way of telling them apart, once you do that it's easy to see the other differences.
Great tits have greyish wings, while Blue tits are blue.
Hope this helped. =]
All of the above plus great tits are bigger and have a black bib. The easy rule is to go for the Black on great tits. Blue tits dont have as much of it. Have a look in a bird book and aquaint yourself. In the mean time.....
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They are both lovely birds
Not when you get them in the hand they arn't!! Blue Tits are especially devious and dont half hurt when they bite!
and dont half hurt when they bite!
Not surprising - have you seen the size of their teeth? 
Not surprising - have you seen the size of their teeth?
I haven't seen the size of their teeth. How big are they?
and dont half hurt when they bite!
Not surprising - have you seen the size of their teeth? 
Are they as scarce as hen's teeth
?
Kingfisher
Not surprising - have you seen the size of their teeth?
I haven't seen the size of their teeth. How big are they?
I sincerely hope this was sent 'tongue in cheek'!!
What does that mean?? (tounge in cheeck)
What does that mean?? (tounge in cheeck)
I mean 'are you joking' or 'do you mean that in a joking way'. You do realize that Blue Tits dont actually have teeth and Wild Canon was only being funny when he said that they had?
Ok oh then. I am still learning you see...