Your children are playing happily. Suddenly, you hear them yelling and notice that they both grab on to the same toy and fight over it.
You don't know who "had it first" and, thus, has some kind of "moral right" to play with it. They both can't talk yet (or have just started to talk), so you can't ask them what happened.
How do you react? Take away the toy (which might be unfair to the child who "had it first" and played with it)? Force them to play together (how, with toddlers)?
(Let's assume that it is not possible to get a second copy of the toy in a feasible amount of time, i.e., in the few minutes until their attention shifts to something else.)