She watched the three equal droplets fall from the end of her toothbrush. 

She was in her school uniform - she couldn't get toothpaste on herself!

She looked down at the floor.

And breathed a sigh of relief when she saw two little puddles on the floor, knowing none of the droplets had landed on her.

How could she be certain she hadn't got hit by any?

~SPOILER BELOW, DON'T SCROLL IF YOU DON'T WANT THE ANSWER YET~

One of the little puddles on the floor was twice the size of the other one because two droplets landed in the same place - all three equally sized droplets had landed on the floor.