Whenever you receive a small act of kindness from a stranger,  don’t just  Thank him/her and forget about it, be equaly kind and help some other stranger. Helping does not mean you have to spend something.  Look around you right now and we will realize that so many people needs you in so many different ways !!  That is how kindness spreads and this is the only way to make this world a better place. After all it is simple mathematics  that if kindness spreads this way then it will again come back to you .  So basically you are getting back what you have done .