hobby-horse
/ˈhɔbɪ hɔ:s; `hɑbɪˌhɔrs/ n
1 long stick with a horse's head, used as a toy 马头杆子（玩具）; 竹马.
2 subject that a person likes to discuss; favourite topic of conversation （讨论、 交谈中）喜爱的话题
You've got me onto (ie talking about) one of my favourite hobby-horses. 你把我引到我喜爱的一个话题上来了.