If you ride two way trails, please take the time to learn proper trail etiquette in terms of hand signals. When an on-coming rider puts up two fingers, they're not flashing you the peace sign. They are communicating to you that there are two more riders coming behind them, so exercise caution!
For example, if you are riding in a group of three people, the leader should flash on-coming riders two fingers, the 2nd guy one finger (middle finger not advisable), and finally the last guy should flash a closed fist (no more riders in your group).
Another, less complicated approach (especially for larger groups) is instead of trying to remember how many riders in your group that are behind you and having to hold up that many fingers (if you even have enough digits), instead just motion your thumb behind you. Then, the sweeping rider in your group can display a closed fist.
This solves a couple of issues; only the lead and sweep need to do anything, AND there is no room for error (lead just has to point his thumb, doesn't need to count....this is particularly useful if a few riders catch up to your group).
Nobody wants a head on collision and hand signals are one of the ways to help minimize accidents. So, be sure to talk about these techniques with your crew.
The next time some flashes you these hand signals, please don't wave or give them a thumbs-up. Respond with the appropriate hand signals.
Have fun & stay safe!