Lock and key is typical example I have observed
If you lock something and forgets the key, that becomes really terrible.
Keys are also useless without their locks.
Loosing a key is horrible to everyone including me as the key locks everything important
But together, they makes great invention.
So I ask, what two things are terrible when separate but great when you put them together??
Milk and coffee.
Just kidding, coffee is always terrible.
Sodium and Chlorine to make it as a Salt - Chemistry lesson