There's currently some kind of solid filling between the tiles in my kitchen and the hard wood in my living room. During the summer when it gets hot and humid, the hard wood expand and cracks the solid filling. After a couple of years, there's not much left of the filling. Here's how it looks:
That filling is also used to connect hard floor from living room to my bedroom:
What should I use to re-fill those gaps so that when summer come, the filling doesn't crack and break? I'm fairly new to DIY, so links to the mats and tools needed are much appreciated.