Hi, 1-In the 1st sentence the Non-defining relative clause( we use it to add more info or details not essential or important) is the sentence '' which I really enjoyed'' - 'which' here is a relative pronoun - So to add some details or more information you use it. So the meaning of the sentence doesn't change (The science lesson took place in the school laboratory.) PS.using the comma makes the difference 2- In the 2nd sentence the Defining relative clause is the same 'which I really enjoyed'. Here you don't add or give more details because the information here is essential ( in this sentence you want to emphasise on your enjoyment)

I agree with syraxes. The first sentence is like saying: "The science lesson was in the laboratory and I really liked it" (meaning there was only one lesson). The second sentence is like saying: "Out of all the science lessons, I enjoyed the one we had in the science in laboratory" (meaning there was more than one different kind of lesson).

a: There was only one science lesson. You enjoyed it. b: There were several science lessons. You enjoyed only one of them. And the lesson that you liked took place in the school laboratory. The other ones took place in some other place (perhaps in a simple room, not in the lab).